cvs-migration documentation update

cvs-migration documentation update

Here's some changes to the cvs-migration.txt.  As usual, in my attempt
to make things clearer someone may have found I've made them less so, or
I may have just gotten something wrong; so any review is welcomed.

I can break up this sort of thing into smaller steps if preferred, the
monolothic patch is just a bit simpler for me for this sort of
thing.

I moved the material describing shared repository management from
core-tutorial.txt to cvs-migration.txt, where it seems more appropriate,
and combined two sections to eliminate some redundancy.

I also revised the earlier sections of cvs-migration.txt, mainly trying
to make it more concise.

I've left the last section of cvs-migration.txt (on CVS annotate
alternatives) alone for now.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>