Mention explicitly when pytest.skip and pytest.xfail can be called

[nicoddemus]

Fix #3219

[coveralls]

Coverage increased (+0.02%) to 94.062% when pulling a13c6a8 on nicoddemus:skip-xfail-docs-3219 into dfa7131 on pytest-dev:master.

[codecov]

Codecov Report

Merging #3922 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3922      +/-   ##
==========================================
- Coverage   94.98%   94.86%   -0.12%     
==========================================
  Files         108      108              
  Lines       23361    23361              
  Branches     2324     2324              
==========================================
- Hits        22189    22162      -27     
- Misses        881      904      +23     
- Partials      291      295       +4

Flag	Coverage Δ
#doctesting	28.59% <ø> (+0.06%)	⬆️
#nobyte	89.99% <ø> (-0.02%)	⬇️
#numpy	28.24% <ø> (+0.06%)	⬆️
#pexpect	47.87% <ø> (ø)	⬆️
#pluggymaster	93.72% <ø> (+0.03%)	⬆️
#py27	93.02% <ø> (ø)	⬆️
#py34	92.25% <ø> (+0.03%)	⬆️
#py35	92.27% <ø> (+0.03%)	⬆️
#py36	93.01% <ø> (-0.26%)	⬇️
#py37	92.43% <ø> (+0.03%)	⬆️
#trial	31.19% <ø> (+0.06%)	⬆️
#xdist	90.12% <ø> (-3.09%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/outcomes.py	95.89% <ø> (ø)	⬆️
testing/test_terminal.py	97.14% <0%> (-2.04%)	⬇️
testing/test_junitxml.py	96.09% <0%> (-1.78%)	⬇️
testing/deprecated_test.py	98.91% <0%> (-1.09%)	⬇️
testing/acceptance_test.py	97.13% <0%> (-0.89%)	⬇️
testing/test_parseopt.py	93.82% <0%> (-0.83%)	⬇️

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