To say that HIERARCH cards make use of COMMENT keywords is innacurate.  Rather, they use commentary keywords which are syntactic feature of the format reserved primarily for COMMENT and HISTORY keywords.  But HIERARCH itself is a commentary keyword albeit not one specifically mentioned by the standard.
Merge pull request #30 from embray/comment-commentary

  Correction on HEIRARCH
Fix another instance of this mistake
Merge pull request #31 from embray/comment-commentary

Followup to #30 Fix another instance of this mistake
Added missing article
Reword second sentence in byte ordering subsection
Use small caps for cfitsio for consistency