Implement ireg and irsync help text for new sha256 file hash feature.
Indicates the various build options & refers to wiki page for more info,
similar to what was done for iput recently (r5646).


git-svn-id: svn://irodssvn.ucsd.edu/trunk@5665 ea137bd6-aa86-47b6-b90d-b101d9f19a90