Extend commands' docs

[tiangolo]

📝 Extend commands' docs for callbacks and one vs multiple commands.

[codecov-io]

Codecov Report

Merging #29 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   99.95%   99.96%   +<.01%     
==========================================
  Files         135      147      +12     
  Lines        2388     2583     +195     
==========================================
+ Hits         2387     2582     +195     
  Misses          1        1

Impacted Files	Coverage Δ
docs/src/commands/callback/tutorial002.py	100% <100%> (ø)
...al/test_commands/test_callback/test_tutorial003.py	100% <100%> (ø)
...al/test_commands/test_callback/test_tutorial004.py	100% <100%> (ø)
docs/src/commands/callback/tutorial004.py	100% <100%> (ø)
docs/src/commands/callback/tutorial001.py	100% <100%> (ø)
docs/src/commands/one_or_multiple/tutorial001.py	100% <100%> (ø)
...al/test_commands/test_callback/test_tutorial001.py	100% <100%> (ø)
...al/test_commands/test_callback/test_tutorial002.py	100% <100%> (ø)
..._commands/test_one_or_multiple/test_tutorial002.py	100% <100%> (ø)
..._commands/test_one_or_multiple/test_tutorial001.py	100% <100%> (ø)
... and 14 more

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