Document extension receiver and scoped-modifier ordering diagnostics

Three related diagnostics are acknowledged only in the catch-all article and need real homes: `CS9347` (static members can't access an extension parameter's value), `CS9349` (`scoped` can't follow `in`/`out`/`ref`/`readonly`), and `CS9350` (a modifier can't immediately follow `scoped`). Together they explain declaration-shape rules for extension members and scoped/ref-style modifiers.

Document `CS9347` in `extension-declarations.md`, keeping examples narrowly focused on extension receiver/value semantics. Document `CS9349` and `CS9350` together in `ref-modifiers-errors.md` so modifier-ordering rules are explained once rather than split across two entries. Move all three out of catch-all-only status. See [Roslyn error codes](https://github.com/dotnet/roslyn/blob/main/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs#L2448-L2452) for exact wording.

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[Associated WorkItem - 632363](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632363)