send-email docs: de-duplicate configuration sections

send-email docs: de-duplicate configuration sections

De-duplicate the discussion of "send-email" configuration, such that
the "git-config(1)" manual page becomes the source of truth, and
"git-send-email(1)" includes the relevant part.

Most commands that suffered from such duplication had diverging text
discussing the same variables, but in this case some config was also
only discussed in one or the other.

This is mostly a move-only change, the exception is a minor rewording
of changing wording like "see above" to "see linkgit:git-config[1]",
as well as a clarification about the big section of command-line
option tweaking config being discussed in git-send-email(1)'s main
docs.

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>