1. There is now only one RELNOTESng stylesheet; the architecture-specific
stylesheets (to handle different values of the arch= attribute) are gone.
2. Several Makefile variable definitions were factored into
doc.relnotes.mk.
Submitted by: dd
o Define a RELN_ROOT variable which points to the root of the relnotes
tree (i.e., src/release/doc).
o By default, define DOC_PREFIX in terms of RELN_ROOT; this gives a
bigger chance of finding the doc/ tree without help in the form of
setting DOC_PREFIX on the command line.
o Respect DOCDIR; `make install` works now.
Approved by: bmah
