Docs: add Database section to built-in-functions.md and nuances to known-limitations.md

- Add ### Database section with all 12 D-functions (DAVERAGE, DCOUNT, DCOUNTA,
  DGET, DMAX, DMIN, DPRODUCT, DSTDEV, DSTDEVP, DSUM, DVAR, DVARP) to
  built-in-functions.md in alphabetical order between Date and Engineering
- Add Database to TOC and remove "database" from "yet to be supported" intro
- Add nuance notes for DGET (#VALUE!/#NUM! error semantics) and
  DMAX/DMIN/DPRODUCT (return 0 when no records match)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marcin-kordas-hoc

docs/guide/built-in-functions.md

@@ -28,6 +28,7 @@ The latest version of HyperFormula has an extensive collection of
 **{{ $page.functionsCount }}** functions grouped into categories:
 
 - [Array manipulation](#array-manipulation)
+- [Database](#database)
 - [Date and time](#date-and-time)
 - [Engineering](#engineering)
 - [Information](#information)
@@ -40,8 +41,7 @@ The latest version of HyperFormula has an extensive collection of
 - [Statistical](#statistical)
 - [Text](#text)
 
-_Some categories such as compatibility, cube, and database are yet to be
-supported._
+_Some categories such as compatibility and cube are yet to be supported._
 
 ::: tip
 You can modify the built-in functions or create your own, by adding a [custom function](custom-functions).
@@ -91,6 +91,23 @@ Total number of functions: **{{ $page.functionsCount }}**
 | YEAR             | Returns the year as a number according to the internal calculation rules.                                                                                                                                                               | YEAR(Number)                                        |
 | YEARFRAC         | Computes the difference between two date values, in fraction of years.                                                                                                                                                                  | YEARFRAC(Date2, Date1[, Format])                    |
 
+### Database
+
+| Function ID | Description                                                                                                     | Syntax                            |
+|:------------|:----------------------------------------------------------------------------------------------------------------|:----------------------------------|
+| DAVERAGE    | Returns the average of all values in a database field that match the given criteria.                            | DAVERAGE(Database, Field, Criteria) |
+| DCOUNT      | Counts the cells containing numbers in a database field that match the given criteria.                          | DCOUNT(Database, Field, Criteria)   |
+| DCOUNTA     | Counts the non-empty cells in a database field that match the given criteria.                                   | DCOUNTA(Database, Field, Criteria)  |
+| DGET        | Returns the single value from a database field that matches the given criteria. Returns #VALUE! if no records match, and #NUM! if more than one record matches. | DGET(Database, Field, Criteria) |
+| DMAX        | Returns the maximum value in a database field that matches the given criteria.                                  | DMAX(Database, Field, Criteria)     |
+| DMIN        | Returns the minimum value in a database field that matches the given criteria.                                  | DMIN(Database, Field, Criteria)     |
+| DPRODUCT    | Returns the product of all values in a database field that match the given criteria.                            | DPRODUCT(Database, Field, Criteria) |
+| DSTDEV      | Returns the sample standard deviation of all values in a database field that match the given criteria.          | DSTDEV(Database, Field, Criteria)   |
+| DSTDEVP     | Returns the population standard deviation of all values in a database field that match the given criteria.      | DSTDEVP(Database, Field, Criteria)  |
+| DSUM        | Returns the sum of all values in a database field that match the given criteria.                                | DSUM(Database, Field, Criteria)     |
+| DVAR        | Returns the sample variance of all values in a database field that match the given criteria.                    | DVAR(Database, Field, Criteria)     |
+| DVARP       | Returns the population variance of all values in a database field that match the given criteria.                | DVARP(Database, Field, Criteria)    |
+
 ### Engineering
 
 | Function ID | Description                                                                         | Syntax                                     |

docs/guide/known-limitations.md

@@ -38,3 +38,5 @@ you can't compare the arguments in a formula like this:
 * The INDEX function doesn't support returning whole rows or columns of the source range – it always returns the contents of a single cell.
 * The FILTER function accepts either single rows of equal width or single columns of equal height. In other words, all arrays passed to the FILTER function must have equal dimensions, and at least one of those dimensions must be 1.
 * Array-producing functions (e.g., SEQUENCE, FILTER) require their output dimensions to be determinable at parse time. Passing cell references or formulas as dimension arguments (e.g., `=SEQUENCE(A1)`) results in a `#VALUE!` error, because the output size cannot be resolved before evaluation.
+* DGET returns `#VALUE!` when no records match the criteria, and `#NUM!` when more than one record matches.
+* DMAX, DMIN, and DPRODUCT return `0` when no records match the criteria (consistent with Excel behavior).