add documentation of register_assert_rewrite

[hjwp]

wip. cf https://twitter.com/nicoddemus/status/1044314722332561408

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs (you can delete this text from the final description, this is
just a guideline):

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS in alphabetical order;

[nicoddemus]

Thanks a lot @hjwp!

[codecov]

Codecov Report

Merging #4032 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4032      +/-   ##
==========================================
+ Coverage   94.31%   94.45%   +0.14%     
==========================================
  Files         109      109              
  Lines       23788    23783       -5     
  Branches     2357     2354       -3     
==========================================
+ Hits        22435    22465      +30     
+ Misses       1033     1005      -28     
+ Partials      320      313       -7

Flag	Coverage Δ
#doctesting	28.58% <ø> (+0.06%)	⬆️
#linux	93.51% <ø> (-0.8%)	⬇️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.18% <ø> (+0.04%)	⬆️
#pexpect	?
#py27	92.07% <ø> (-0.4%)	⬇️
#py34	92.11% <ø> (+0.15%)	⬆️
#py35	91.48% <ø> (-0.49%)	⬇️
#py36	92.68% <ø> (+0.2%)	⬆️
#py37	91.65% <ø> (-0.47%)	⬇️
#trial	31.21% <ø> (+0.19%)	⬆️
#windows	93.8% <ø> (?)
#xdist	18.53% <ø> (+0.04%)	⬆️

Impacted Files	Coverage Δ
src/_pytest/doctest.py	94.37% <0%> (-2.41%)	⬇️
testing/python/integration.py	89.36% <0%> (-2.13%)	⬇️
src/_pytest/assertion/rewrite.py	95.18% <0%> (-0.17%)	⬇️
testing/code/test_source.py	96.79% <0%> (-0.01%)	⬇️
src/_pytest/fixtures.py	97.27% <0%> (+0.27%)	⬆️
testing/test_capture.py	99.24% <0%> (+0.3%)	⬆️
src/_pytest/pytester.py	86.01% <0%> (+0.45%)	⬆️
testing/acceptance_test.py	97.8% <0%> (+0.65%)	⬆️
src/_pytest/nodes.py	94.53% <0%> (+0.84%)	⬆️
testing/code/test_code.py	95.45% <0%> (+1.33%)	⬆️
... and 5 more

---

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[coveralls]

Coverage increased (+0.02%) to 93.811% when pulling b03bad5 on hjwp:patch-1 into 4474bee on pytest-dev:master.