Update docs for 'pytest_cmdline_parse' hook to note availability limi…

[garytyler]

fixes #4974

[nicoddemus]

Thanks a lot for the contribution, appreciate it!

[codecov]

Codecov Report

Merging #5010 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5010      +/-   ##
==========================================
- Coverage   95.68%   95.44%   -0.24%     
==========================================
  Files         113      113              
  Lines       25178    25178              
  Branches     2498     2498              
==========================================
- Hits        24092    24032      -60     
- Misses        765      813      +48     
- Partials      321      333      +12

Impacted Files	Coverage Δ
src/_pytest/hookspec.py	100% <ø> (ø)	⬆️
testing/test_pathlib.py	91.17% <0%> (-8.83%)	⬇️
testing/test_tmpdir.py	95.16% <0%> (-3.77%)	⬇️
testing/deprecated_test.py	96.77% <0%> (-3.23%)	⬇️
src/_pytest/capture.py	91.23% <0%> (-3.15%)	⬇️
src/_pytest/compat.py	93.96% <0%> (-3.02%)	⬇️
src/_pytest/pathlib.py	88.46% <0%> (-2.2%)	⬇️
src/_pytest/junitxml.py	94.56% <0%> (-1.28%)	⬇️
testing/acceptance_test.py	96.78% <0%> (-1.08%)	⬇️
src/_pytest/config/findpaths.py	94% <0%> (-1%)	⬇️
... and 10 more

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