Modify the release-building process so the srelease distribution

Modify the release-building process so the srelease distribution
doesn't get spammed by RELNOTESng files.

Previously, we built the RELNOTESng files "in place" and then
populated the media (e.g. floppy, cdrom, ftp) areas from the build
directories.  However, the distributions got rolled after the
RELNOTESng build, and thus picked up the rendered RELNOTESng files.

Now, we install the RELNOTESng documents to R/stage/release.doc, then
clean out the original directories in src/release/doc.  We use the
renditions in the staging area to populate the media areas.

PR:		33141
MFC after:	3 days
		(plus re approval)