doc(mixin): clarify precedence rules and document limitations

[fredbi]

Rewrite the Mixin godoc into three labeled sections:

• Argument order and precedence, with a worked host example showing that the primary always wins on collision and that mixins are listed in decreasing priority order.
• What gets merged, enumerating exactly which fields are filled from empty primaries, which are merged entry-by-entry with collision warnings, and which are unioned silently (schemes, consumes, produces).
• Notes and limitations, with cross-references to the new goswagger.io FAQ entries on YAML anchor preservation and on output ordering.

This is a documentation-only change addressing user-confusion issues in go-swagger/go-swagger:

• Mixin go-swagger/go-swagger#1823 (precedence)
• Swagger mixin with anchors defined in another file. go-swagger/go-swagger#1928 (YAML anchors not preserved)
• Operations sorting order for the "mixin" in CLI go-swagger/go-swagger#2130 (output is alphabetically ordered; this is an architectural constraint inherited from the underlying spec model, not fixable here)

Companion FAQ entries land in go-swagger/go-swagger docs/faq/faq_swagger.md and the canonical reference moves into docs/usage/mixin.md.

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.01%. Comparing base (c2f261a) to head (66496b1).
⚠️ Report is 11 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #196   +/-   ##
=======================================
  Coverage   92.01%   92.01%           
=======================================
  Files          30       30           
  Lines        3394     3394           
=======================================
  Hits         3123     3123           
  Misses        189      189           
  Partials       82       82           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.