Unionfs: document cache-coherency design and implementation

Unionfs: document cache-coherency design and implementation

Document our cache-coherency design, implementation; also mention alternate
designs and a couple of limitations which might be overcome by special
kernel support at the VFS/MM levels.  Now that cache-coherency is working,
remove the now-obsolete older discussion of temporary workarounds.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>