Clarify docs for pytest.raises match.

[peterschutt]

Fix #5208.

Document explicit behavior of match and brief note on how to handle matching a string that may contain special re chars.

[nicoddemus]

Thanks a lot @5uper5hoot!

[codecov]

Codecov Report

Merging #5223 into master will decrease coverage by 0.97%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5223      +/-   ##
==========================================
- Coverage   96.06%   95.09%   -0.98%     
==========================================
  Files         114      114              
  Lines       25824    25824              
  Branches     2550     2550              
==========================================
- Hits        24809    24558     -251     
- Misses        705      944     +239     
- Partials      310      322      +12

Impacted Files	Coverage Δ
src/_pytest/python_api.py	97.48% <ø> (ø)	⬆️
testing/test_argcomplete.py	20.28% <0%> (-47.83%)	⬇️
src/_pytest/_argcomplete.py	33.33% <0%> (-41.67%)	⬇️
testing/test_conftest.py	90.4% <0%> (-9.23%)	⬇️
testing/test_pathlib.py	91.17% <0%> (-8.83%)	⬇️
src/_pytest/pytester.py	85.71% <0%> (-5.4%)	⬇️
testing/test_collection.py	95.29% <0%> (-4.52%)	⬇️
testing/test_tmpdir.py	94.62% <0%> (-4.31%)	⬇️
src/_pytest/assertion/util.py	93.38% <0%> (-4.29%)	⬇️
testing/test_parseopt.py	94.73% <0%> (-3.24%)	⬇️
... and 14 more

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[nicoddemus]

Linting passed, merging. 👍

[wimglenn]

Good to see this was clarified better, it was one of my gripes in #4538