Show the matching trait in the struct type macro docstrings

[quinnj]

The docstrings generated for @Struct, @Mutable, @DictType, and the other struct type macros all said the macro is equivalent to StructTypes.StructType(::Type{MyStruct}) = StructType.Struct(). That example names a nonexistent StructType module and shows the wrong trait for every macro except @Struct.

The example now interpolates the macro's own trait, so @doc StructTypes.@Mutable shows StructTypes.StructType(::Type{MyStruct}) = StructTypes.Mutable().

Verification: rendered @doc for @Mutable, @Struct, and @DictType on Julia 1.12.6 and checked both examples in each show the matching StructTypes.<Trait>(). Docstring-only change.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.38%. Comparing base (90f9ecf) to head (bc303f8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #108   +/-   ##
=======================================
  Coverage   78.38%   78.38%           
=======================================
  Files           2        2           
  Lines         421      421           
=======================================
  Hits          330      330           
  Misses         91       91           

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