Glossary: Add an entry for "Plain Access"

The book uses "plain access", so it deserves a glossary entry.
And of course "marked access" is its antonym, more or less.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
[akiyks: rebased on indexing-scheme branch]
1 file changed