Document extension receiver and scoped-modifier ordering diagnostics

[BillWagner]

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 for exact wording.

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Associated WorkItem - 632363