Adds note on multiple module marker usage

[0bsearch]

Small explicit note on using multiple markers on module level.

[codecov]

Codecov Report

Merging #4433 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4433      +/-   ##
==========================================
- Coverage    95.9%   95.89%   -0.01%     
==========================================
  Files         111      111              
  Lines       25008    25008              
  Branches     2438     2438              
==========================================
- Hits        23984    23982       -2     
- Misses        723      724       +1     
- Partials      301      302       +1

Flag	Coverage Δ
#docs	30.17% <ø> (+0.07%)	⬆️
#doctesting	30.17% <ø> (+0.07%)	⬆️
#linting	30.17% <ø> (+0.07%)	⬆️
#linux	95.72% <ø> (-0.01%)	⬇️
#nobyte	92.65% <ø> (ø)	⬆️
#numpy	93.44% <ø> (ø)	⬆️
#pexpect	41.9% <ø> (ø)	⬆️
#py27	94.02% <ø> (ø)	⬆️
#py34	92.16% <ø> (+0.06%)	⬆️
#py35	92.18% <ø> (+0.06%)	⬆️
#py36	92.2% <ø> (+0.06%)	⬆️
#py37	94.1% <ø> (-0.01%)	⬇️
#trial	93.44% <ø> (ø)	⬆️
#windows	94.1% <ø> (ø)	⬆️
#xdist	93.95% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/capture.py	93.42% <0%> (-0.46%)	⬇️

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