docs(metadata): document getAvailableFunctions/getFunctionDetails (HF-249)

Add a CHANGELOG entry for the new built-in function metadata API, and a
"Function metadata" section to the custom functions guide clarifying that the
catalogue covers built-ins only — custom functions are absent from
getAvailableFunctions() and getFunctionDetails() returns undefined for them.

Author: marcin-kordas-hoc

CHANGELOG.md

@@ -9,6 +9,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Added
 
+- Added the `getAvailableFunctions()` and `getFunctionDetails()` methods (both static and instance) for retrieving built-in function metadata: category, translated name, short description, generated syntax, and parameters.
 - Added an Indonesian (Bahasa Indonesia) language pack. [#1674](https://github.com/handsontable/hyperformula/pull/1674)
 
 ## [3.3.0] - 2026-05-20

docs/guide/custom-functions.md

@@ -538,3 +538,29 @@ MyCustomPlugin.translations = {
 };
 ```
 :::
+
+## Function metadata
+
+HyperFormula ships metadata for its built-in functions (category, translated
+name, short description, syntax, and parameters). You can retrieve it with the
+`getAvailableFunctions()` and `getFunctionDetails()` methods, available both as
+static methods and as instance methods:
+
+```js
+// a short list of all built-in functions, with names translated for a language
+const functions = HyperFormula.getAvailableFunctions('enGB');
+
+// the full details of a single built-in function
+const sumDetails = HyperFormula.getFunctionDetails('SUM', 'enGB');
+```
+
+This metadata covers **built-in functions only**. Custom functions are not
+included: they don't appear in the `getAvailableFunctions()` list, and
+`getFunctionDetails()` returns `undefined` for them.
+
+```js
+HyperFormula.registerFunctionPlugin(MyCustomPlugin, MyCustomPlugin.translations);
+
+// a custom function is not part of the built-in metadata
+const details = HyperFormula.getFunctionDetails('MY_FUNCTION', 'enGB'); // undefined
+```