docs: update apps/appsets in any namespace flags documentation

[nitishfy]

Based on last conversation in Argo CD Contributor meeting, it is better to document the namespace feature flags to avoid any sort of confusion.
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Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 55.97%. Comparing base (a1f90b5) to head (b907d5a).
Report is 444 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argocd/commands/admin/backup.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #22480      +/-   ##
==========================================
+ Coverage   55.92%   55.97%   +0.05%     
==========================================
  Files         343      343              
  Lines       57429    57439      +10     
==========================================
+ Hits        32119    32154      +35     
+ Misses      22657    22641      -16     
+ Partials     2653     2644       -9     

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