Document newer regex features as not being transformed

[alangpierce]

Fixes #434

[codecov-io]

Codecov Report

Merging #442 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #442   +/-   ##
=======================================
  Coverage   81.22%   81.22%           
=======================================
  Files          50       50           
  Lines        5157     5157           
  Branches     1204     1204           
=======================================
  Hits         4189     4189           
  Misses        673      673           
  Partials      295      295

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