Added info to access exitcodes directly

[yashtodi94]

Relates to:

1. pytest#5125
2. pytest#5420

Better visibility on usage of exitcodes.

[codecov]

Codecov Report

Merging #5694 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #5694     +/-   ##
=========================================
+ Coverage   95.91%   96.11%   +0.2%     
=========================================
  Files         117      117             
  Lines       25823    25823             
  Branches     2497     2497             
=========================================
+ Hits        24767    24820     +53     
+ Misses        748      699     -49     
+ Partials      308      304      -4

Impacted Files	Coverage Δ
src/_pytest/terminal.py	93.02% <0%> (+0.42%)	⬆️
testing/test_assertrewrite.py	84.68% <0%> (+1.41%)	⬆️
src/_pytest/pytester.py	90.96% <0%> (+5.92%)	⬆️

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[asottile]

thanks!

[asottile]

oof, bad rng you got the creepy not-cat