doc: add deprecated field to docs

Signed-off-by: Niels Pardon <par@zurich.ibm.com>

Author: nielspardon

site/docs/expressions/scalar_functions.md

@@ -12,6 +12,7 @@ A function is a scalar function if that function takes in values from a single r
 | Nullability Handling | Describes how nullability of input arguments maps to nullability of output arguments. Three options are: `MIRROR`, `DECLARED_OUTPUT` and `DISCRETE`. More details about nullability handling are listed below.                                                                                                                                                                                        | Optional, defaults to `MIRROR` |
 | Description            | Additional description of function for implementers or users. Should be written human-readable to allow exposure to end users. Presented as a map with language => description mappings. E.g. `{ "en": "This adds two numbers together.", "fr": "cela ajoute deux nombres"}`.                                                                                                                         | Optional                            |
 | Return Value | The output type of the expression.  Return types can be expressed as a fully-defined type or a type expression. See below for more on type expressions.                                                                                                                                                                                                                                               | Required                            |
+| deprecated | Indicates whether this function or function signature has been deprecated. | Optional, defaults to false |
 | Implementation Map     | A map of implementation locations for one or more implementations of the given function. Each key is a function implementation type. Implementation types include examples such as: AthenaArrowLambda, TrinoV361Jar, ArrowCppKernelEnum, GandivaEnum, LinkedIn Transport Jar, etc. [Definition TBD]. Implementation type has one or more properties associated with retrieval of that implementation. | Optional                            |