Allow documenting function via comment on DeclareGlobalName

[fingolfin]

Keep DeclareGlobalName entries documented as variables by default,
but promote them to functions when @arguments or @returns provides
function-style documentation. Preserve explicit @ItemType
overrides and document the resulting behavior in the manual and
regression fixtures.

AI assistance disclosure: Codex assisted with implementing the
parser change, updating the manual text, and revising the test
fixtures and expected output.

Co-authored-by: Codex codex@openai.com

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.61%. Comparing base (a04aa09) to head (5d36023).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/Parser.gi	78.57%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
- Coverage   91.77%   91.61%   -0.16%     
==========================================
  Files          13       13              
  Lines        2566     2589      +23     
==========================================
+ Hits         2355     2372      +17     
- Misses        211      217       +6     

Files with missing lines	Coverage Δ
gap/Parser.gi	96.20% <78.57%> (-0.56%)	⬇️

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