Document that System.Text.Json ignores [EnumMember]

[RaphaelFakhri]

Fixes #3328.

Adds a Rename Enum Values in Schemas section to the SwaggerGen docs, covering the difference you agreed was worth writing down: System.Text.Json does not implement [EnumMember], Newtonsoft.Json does, and [JsonStringEnumMemberName] is the native option.

I verified all three claims against master rather than trusting the issue, by generating the schema for the same enum through each resolver:

Serializer	Attribute	enum in the generated schema
System.Text.Json	[EnumMember(Value = "X-foo")]	Value1, X
System.Text.Json	[JsonStringEnumMemberName("X-foo")]	Value1, X-foo
Newtonsoft.Json	[EnumMember(Value = "X-foo")]	Value1, X-foo

The C# examples are plain fenced blocks rather than MarkdownSnippets snippets. JsonStringEnumMemberNameAttribute is .NET 9 and later, and DocumentationSnippets multi-targets down to net8.0, so a compiled snippet would need an #if NET9_0_OR_GREATER around it that would then show up in the rendered page. Happy to switch it to a snippet with the guard, or to a net9.0-and-later snippet project, if you would rather have it compiled.

markdownlint-cli2 with the repo's .markdownlint.json reports 0 issues, and dotnet mdsnippets leaves the file unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.24%. Comparing base (d0b1e26) to head (3812750).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4127   +/-   ##
=======================================
  Coverage   95.24%   95.24%           
=======================================
  Files         111      111           
  Lines        4142     4142           
  Branches      848      848           
=======================================
  Hits         3945     3945           
  Misses        197      197           

Flag	Coverage Δ
Linux	95.24% <ø> (ø)
Windows	95.24% <ø> (ø)
macOS	95.24% <ø> (ø)

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[RaphaelFakhri]

Agreed. The section is now four lines: one sentence on why the serializer decides the names, and links to the upstream documentation for [JsonStringEnumMemberName] (System.Text.Json) and for [EnumMember] with StringEnumConverter (Newtonsoft.Json). The examples and sample JSON are removed.