T3566,T3546: added a blurb in usage.rst for usage of flag -r

[dhirensr]

PR for #3546 (comment) and #3566

[coveralls]

Coverage increased (+0.01%) to 94.061% when pulling f3b0caf on dhirensr:docs_for_detailed_info into 019e33e on pytest-dev:master.

[codecov]

Codecov Report

Merging #3917 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3917      +/-   ##
==========================================
+ Coverage   96.24%   96.24%   +<.01%     
==========================================
  Files         108      108              
  Lines       23353    23353              
==========================================
+ Hits        22475    22476       +1     
+ Misses        878      877       -1

Flag	Coverage Δ
#doctesting	31.93% <ø> (+0.05%)	⬆️
#nobyte	93.5% <ø> (-0.01%)	⬇️
#numpy	31.48% <ø> (+0.05%)	⬆️
#pexpect	53.58% <ø> (ø)	⬆️
#pluggymaster	95.09% <ø> (+0.02%)	⬆️
#py27	94.61% <ø> (ø)	⬆️
#py34	93.9% <ø> (+0.02%)	⬆️
#py35	93.9% <ø> (+0.02%)	⬆️
#py36	94.82% <ø> (ø)	⬆️
#py37	94.07% <ø> (+0.02%)	⬆️
#trial	34.15% <ø> (+0.05%)	⬆️
#xdist	94.64% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/capture.py	93.59% <0%> (+0.22%)	⬆️

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[nicoddemus]

Thanks a lot @dhirensr, great work!

I've moved that section further up the docs and updated the text a bit to be more brief about the description. 👍