Focus of this CL is on the PNaCl records that can appear in bitcode files.
A subsequent CL will add how the records are converted to bitcode sequences.
Replaces CL: https://codereview.chromium.org/364463002
BUG=None
NOTRY=True
(documentation only change).
Review URL: https://codereview.chromium.org/725333002
Cr-Commit-Position: refs/heads/master@{#305041}