Add permissions examples to admin docs

[therealsilence]

• Add permissions examples to Synapse Admin guide.
• Add permissions "best practices" to Synapse Admin guide.
• Add auth.user.allowed command to Storm command reference.
• Minor doc fixes / clarifications.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.10 ⚠️

Comparison is base (dd084b6) 97.34% compared to head (81b77ef) 97.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3187      +/-   ##
==========================================
- Coverage   97.34%   97.24%   -0.10%     
==========================================
  Files         224      224              
  Lines       44840    44840              
==========================================
- Hits        43648    43604      -44     
- Misses       1192     1236      +44     

Flag	Coverage Δ
linux	97.24% <ø> (ø)
linux_replay	?

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see 9 files with indirect coverage changes

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