Improve docs of pytest.importorskip

[timhoffm]

• Link function docstring in description.
• Document return value.
• Add example usage to docstring.

[codecov]

Codecov Report

Merging #5718 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5718      +/-   ##
==========================================
- Coverage   96.16%   96.15%   -0.02%     
==========================================
  Files         117      117              
  Lines       25864    25864              
  Branches     2498     2498              
==========================================
- Hits        24873    24869       -4     
- Misses        690      695       +5     
+ Partials      301      300       -1

Impacted Files	Coverage Δ
src/_pytest/outcomes.py	98.46% <ø> (ø)	⬆️
src/_pytest/doctest.py	94.38% <0%> (-2.25%)	⬇️
src/_pytest/pastebin.py	91.22% <0%> (+3.5%)	⬆️

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[nicoddemus]

Thanks!