Add docs for indexable types to C#

Includes references to changes made by
clockworklabs/SpacetimeDB#2506 ,
which as of writing has not merged.
We should not push this commit live until after that PR is released.

Author: gefjon

docs/modules/c-sharp/index.md

@@ -611,6 +611,17 @@ public partial struct AcademicPaper {
 
 Any table supports getting an [`Index`](#class-index) using `ctx.Db.{table}.{index}`. For example, `ctx.Db.academic_paper.TitleAndDate` or `ctx.Db.academic_paper.Venue`.
 
+## Indexable Types
+
+SpacetimeDB supports only a restricted set of types as index keys:
+
+- Signed and unsigned integers of various widths.
+- `bool`.
+- `string`.
+- [`Identity`](#struct-identity).
+- [`ConnectionId`](#struct-connectionid).
+- `enum`s annotated with [`SpacetimeDB.Type`](#attribute-spacetimedbtype).
+
 ## Class `Index`
 
 ```csharp
@@ -1391,4 +1402,4 @@ Stored in reducer-scheduling tables as a column.
 [`DateTimeOffset`]: https://learn.microsoft.com/en-us/dotnet/api/system.datetimeoffset?view=net-9.0
 [`TimeSpan`]: https://learn.microsoft.com/en-us/dotnet/api/system.timespan?view=net-9.0
 [unix epoch]: https://en.wikipedia.org/wiki/Unix_time
-[`System.Random`]: https://learn.microsoft.com/en-us/dotnet/api/system.random?view=net-9.0
+[`System.Random`]: https://learn.microsoft.com/en-us/dotnet/api/system.random?view=net-9.0