docs: add versioning guidance for spread-in properties using augment decorators

[Copilot]

The versioning docs cover @added/@removed on inline-declared properties but not on properties introduced via model spreads (...SomeModel), which have no inline declaration site for decorators. This gap causes authors to either leave spread properties unscoped (breaking the API surface of earlier versions) or attempt workarounds that don't compile.

Adds a new subsection under "Complex Scenarios" in the Evolving APIs versioning doc covering:

• Why augment decorators are required — spread-in members have no inline site for @added/@removed; the @@ augment form must be used instead
• @@added example — scoping a ManagedServiceIdentityProperty spread to a specific version
• @@removed example — removing a spread-in property in a later version
• Applicability note — same pattern works for @@madeOptional, @@madeRequired, @@renamedFrom, @@typeChangedFrom

model Employee is TrackedResource<EmployeeProperties> {
  ...ResourceNameParameter<Employee>;
  ...ManagedServiceIdentityProperty;
}

// Introduce the spread-in 'identity' property starting in v2 only.
@@added(Employee.identity, Versions.v2);

• Fixes TypeSpec Authoring Docs: cover version-scoping a spread model property with @@added #4809

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