Add missing content to user defined function mapping

[maumar]

• The HasFunction overload which takes lambda with default params to register a function
• DbFunctionAttribute and properties over that. (https://github.com/dotnet/EntityFramework.Docs/issues/2476)
• Specifying a schema or marking function as built-in
• Mapping function return type or parameter to particular store type when not natively supported and scenario where it works.
• argument propagates nullability API and link to the null semantics docs.
• Ideally, we should put down the example of mapping to JSON function dotnet/efcore#11295 (comment) Probably most used usecase for the UDF so far
• Mention that TVF cannot use HasTranslation API
• Few more cautions about HasTranslation API and need to understand the SQL expression tree.

[maumar]

also add something about value converters how they use converter from the argument passed to the function (and that we just need to map it to the store type) - see https://github.com/dotnet/efcore/issues/23498