Add an example on how to update config.warn calls

[nicoddemus]

As commented in pytest-dev/pytest-cov#230 (review)

[codecov]

Codecov Report

Merging #4029 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4029      +/-   ##
=========================================
+ Coverage   94.31%   94.5%   +0.18%     
=========================================
  Files         109     109              
  Lines       23788   23788              
  Branches     2357    2357              
=========================================
+ Hits        22435   22480      +45     
+ Misses       1033    1000      -33     
+ Partials      320     308      -12

Flag	Coverage Δ
#doctesting	29.36% <ø> (+0.84%)	⬆️
#linux	94.35% <ø> (+0.04%)	⬆️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.27% <ø> (+0.13%)	⬆️
#pexpect	0% <ø> (ø)	⬆️
#py27	92.62% <ø> (+0.15%)	⬆️
#py34	92.11% <ø> (+0.15%)	⬆️
#py35	92.12% <ø> (+0.15%)	⬆️
#py36	92.69% <ø> (+0.21%)	⬆️
#py37	92.32% <ø> (+0.2%)	⬆️
#trial	31.3% <ø> (+0.28%)	⬆️
#windows	93.8% <ø> (?)
#xdist	18.57% <ø> (+0.08%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/fixtures.py	97.27% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.24% <0%> (+0.3%)	⬆️
src/_pytest/pytester.py	86.01% <0%> (+0.45%)	⬆️
testing/acceptance_test.py	97.8% <0%> (+0.65%)	⬆️
src/_pytest/nodes.py	94.53% <0%> (+0.84%)	⬆️
src/_pytest/terminal.py	91.6% <0%> (+1.74%)	⬆️
src/_pytest/capture.py	89.93% <0%> (+3.2%)	⬆️
testing/test_tmpdir.py	100% <0%> (+4.93%)	⬆️
src/_pytest/paths.py	100% <0%> (+8.69%)	⬆️
testing/test_paths.py	100% <0%> (+13.63%)	⬆️

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