DOC: Prevent concept checking comments from being parsed by Doxygen

The comments for "Begin/End concept checking" are not API documentation
and should not start with "/**", which triggers parsing by Doxygen.

This fixes a bug where the function definition immediately following the
"End concept checking" line gets improperly documented.

Change-Id: I91d66f3888c505ffebc55a3d4a75b53ac5b9be03
Merge topic 'ConceptChecking-Comments'

17030da DOC: Prevent concept checking comments from being parsed by Doxygen