glossary: expand and clarify some definitions, prune cross-references

Revise and expand some of the definitions in the glossary, based in part
on a recent thread started by a user looking for help with some of the
jargon.  I've borrowed some of the language from Linus's email on that
thread.  (I'm assuming standing permission to plagiarize Linus's
email....)

Also start making a few changes to mitigate the appearance of
"circularity" mentioned in that thread:
	- feel free to use somewhat longer definitions and to explain
	  some things more than once instead of relying purely on
	  cross-references
	- don't use cross-references when they're redundant: eliminate
	  self-references and repeated references to the same entry.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>