top: add additional memory information to the man page

top: add additional memory information to the man page

In response to Issue #21, the commit referred to below
provided some much needed improvements and corrections
to topic `3a. DESCRIPTIONS of Fields' in the man page.
However, it assumed a reader possessed much background
knowledge that may not, in truth, actually be present.

So without, hopefully, insulting anyone's intelligence
this patch offers an expanded discussion of some terms
and concepts within a separate section under OVERVIEW.

[ plus it affords an opportunity to incorporate that ]
[ extremely useful table from Florent Bruneau's post ]

Reference(s):
commit f2a08cf16794ec6085bdecbaf8f7c2887cd4e87f
https://techtalk.intersec.com/2013/07/memory-part-1-memory-types/

Signed-off-by: Jim Warner <james.warner@comcast.net>