add serializer::stream docs

[cmazakas]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.37%. Comparing base (7bd53ed) to head (6c129c4).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #82   +/-   ##
========================================
  Coverage    89.37%   89.37%           
========================================
  Files           77       77           
  Lines         4348     4348           
========================================
  Hits          3886     3886           
  Misses         462      462           

Files	Coverage Δ
include/boost/http_proto/serializer.hpp	97.43% <ø> (ø)

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[cppalliance-bot]

GCOVR code coverage report https://82.http-proto.prtest.cppalliance.org/gcovr/index.html
LCOV code coverage report https://82.http-proto.prtest.cppalliance.org/genhtml/index.html Coverage Diff https://82.http-proto.prtest.cppalliance.org/gcovr/coverage_diff.txt

[cppalliance-bot]

GCOVR code coverage report https://82.http-proto.prtest.cppalliance.org/gcovr/index.html
LCOV code coverage report https://82.http-proto.prtest.cppalliance.org/genhtml/index.html Coverage Diff https://82.http-proto.prtest.cppalliance.org/gcovr/coverage_diff.txt

[vinniefalco]

Can we please get the doc previews working?