add documentation for writing config files

Replace TODO introduced in commit 9c3c22 with documentation
explaining Git config API functions for writing configuration
files.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
git-push: fix link in man page

Signed-off-by: Tony Finch <dot@dotat.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'ta/doc-config'

* ta/doc-config:
  add documentation for writing config files
Merge branch 'tf/maint-doc-push'

* tf/maint-doc-push:
  git-push: fix link in man page
Git 2.1.0-rc1

Signed-off-by: Junio C Hamano <gitster@pobox.com>