[memprof] Update comments for writeMemProf and its helpers (#92446)

This patch adds comments for writeMemProf{V0,V1,V2} in a
version-specific manner.  The mostly repetitive nature of the comments
is somewhat unfortunate but intentional to make it easy to retire
older versions.

Without this patch, the comment just before writeMemProf documents the
Version1 format, which is very confusing.