Updated docs on base backups

Hi all,

See attached changed to current docs on subject. They have clearly
been reworked by multiple people piecemeal and had many issues
which made less than a joy to read, in fact they were damn frustrating
to read.

I've gone through the whole section and rewritten what needed to be.
I think the result is more cohesive, better explained, certainly
more accurate in some ways.

As imperfect as this may be, I think this version is significantly
better then what it replaces. It would be nice if someone had
the patience to read through and do what editing needs to be done,
as well as provide a technical sanity check.

There's a real dilemma facing a newcomer sitting down to rewrite
a portion of the manual -- you can't really consult the documentation to
make sure you've properly understood how the system works...

Amir