Update documentation to indicate a fixture can be invoked more than once in it's scope.

[jeffreyrack]

Closes #4058 by updating the documentation to reference that a scope can be invoked more than once in the scope it's defined for.

[coveralls]

Coverage increased (+0.02%) to 93.811% when pulling c5dec60 on jeffreyrack:4058-update-fixture-docs into 20f93ae on pytest-dev:master.

[coveralls]

Coverage increased (+0.02%) to 93.811% when pulling c5dec60 on jeffreyrack:4058-update-fixture-docs into 20f93ae on pytest-dev:master.

[codecov]

Codecov Report

Merging #4070 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4070      +/-   ##
==========================================
+ Coverage   94.49%   94.53%   +0.04%     
==========================================
  Files         109      109              
  Lines       23837    23837              
  Branches     2361     2361              
==========================================
+ Hits        22524    22534      +10     
+ Misses       1002      996       -6     
+ Partials      311      307       -4

Flag	Coverage Δ
#doctesting	29.31% <ø> (+0.83%)	⬆️
#linux	94.38% <ø> (+0.04%)	⬆️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.22% <ø> (+0.13%)	⬆️
#pexpect	0% <ø> (ø)	⬆️
#py27	92.66% <ø> (+0.04%)	⬆️
#py34	92.11% <ø> (+0.03%)	⬆️
#py35	92.12% <ø> (+0.03%)	⬆️
#py36	92.69% <ø> (+0.04%)	⬆️
#py37	92.33% <ø> (+0.03%)	⬆️
#trial	31.25% <ø> (+0.28%)	⬆️
#windows	93.84% <ø> (+0.58%)	⬆️
#xdist	18.55% <ø> (+0.08%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/terminal.py	91.62% <0%> (+1.74%)	⬆️

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[nicoddemus]

Thanks @jeffreyrack for the contribution!