docs: improve command usage documentation

[phm07]

This PR makes command usage documentation consistent by enforcing docopt rules and also defining and documenting other additional conventions.

Fixes #708

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.01124% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 58.77%. Comparing base (4712950) to head (9457dc5). Report is 2 commits behind head on main.

Files	Patch %	Lines
internal/cmd/base/delete.go	50.00%	1 Missing and 1 partial :warning:
internal/cmd/base/labels.go	0.00%	2 Missing :warning:
internal/cmd/base/set_rdns.go	0.00%	1 Missing :warning:
internal/cmd/base/update.go	0.00%	1 Missing :warning:
internal/cmd/server/enable_protection.go	0.00%	1 Missing :warning:
internal/cmd/server/ssh.go	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #709      +/-   ##
==========================================
- Coverage   58.78%   58.77%   -0.02%     
==========================================
  Files         179      179              
  Lines        6536     6539       +3     
==========================================
+ Hits         3842     3843       +1     
- Misses       2087     2088       +1     
- Partials      607      608       +1     

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