docs: add CONFIGURATION sections that fuzzy map to built-ins

docs: add CONFIGURATION sections that fuzzy map to built-ins

Add a CONFIGURATION section to the documentation of various built-ins,
for those cases where the relevant config/NAME.txt doesn't map only to
one git-NAME.txt. In particular:

 * config/blame.txt: used by git-{blame,annotate}.txt. Since the
   git-annotate(1) documentation refers to git-blame(1) don't add a
   "CONFIGURATION" section to git-annotate(1), only to git-blame(1).

 * config/checkout.txt: maps to both git-checkout.txt and
   git-switch.txt (but nothing else).

 * config/init.txt: should be included in git-init(1) and
   git-clone(1).

 * config/column.txt: We should ideally mention the relevant subset of
   this in git-{branch,clean,status,tag}.txt, but let's punt on it for
   now. We will when we eventually split these sort of files into
   e.g. config/column.txt and
   config/column/{branch,clean,status,tag}.txt, with the former
   including the latter set.

Things that are being left out, and why:

 * config/{remote,remotes,credential}.txt: Configuration that affects
   how we talk to remote repositories is harder to untangle. We'll need
   to include some of this in git-{fetch,remote,push,ls-remote}.txt
   etc., but some of those only use a small subset of these
   options. Let's leave this for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>