First pass at documenting serializer

[willkg]

This documents the html5lib.serializer module for issue #307.

[willkg]

@gsnedders @jgraham Can one of you review this, please?

[codecov-io]

Codecov Report

Merging #376 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   90.78%   90.75%   -0.04%     
==========================================
  Files          50       50              
  Lines        6927     6942      +15     
  Branches     1322     1328       +6     
==========================================
+ Hits         6289     6300      +11     
- Misses        482      483       +1     
- Partials      156      159       +3

Impacted Files	Coverage Δ
html5lib/serializer.py	85.36% <ø> (ø)	⬆️
html5lib/treebuilders/__init__.py	81.81% <0%> (-18.19%)	⬇️

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[willkg]

The codecov report is erroneous. The changes are all docstrings--no code changes at all. We can ignore that.