fix & document description vs help, metavar vs dest

[casperdcl]

• zsh: metavar overrides dest
• zsh & fish
  • description overrides help
  • wrap one-line descriptions
• tcsh: respect help=SUPPRESS
• fix choices: dict without confusing with subparsers (re-fixes shtab fails if option's choices are not sequence [zsh] #214 -> fix: support non-sequence choices in zsh completion #220)
• docs: update FAQs (fixes Subparsers are ignored when help is not defined on the subparser #190, fixes subcommands don't get description but get help anyway #169, fixes [bug] subparser.help cannot be converted to description #105)
• misc tidy

Forward-compatible with python/cpython#67037 (hopefully gets fixed).

/CC @mpkocher @fdev31 @Freed-Wu @paulstretenowich @morphine1 @jkmnt @Sanjays2402

[codecov]

Codecov Report

❌ Patch coverage is 91.42857% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.50%. Comparing base (116e9b4) to head (65518ed).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
+ Coverage   85.13%   87.50%   +2.36%     
==========================================
  Files           3        3              
  Lines         471      488      +17     
  Branches       92       95       +3     
==========================================
+ Hits          401      427      +26     
+ Misses         39       34       -5     
+ Partials       31       27       -4     

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