command-list.txt: add missing 'gitcredentials' and 'gitremote-helpers'

The guides 'gitcredentials' and 'gitremote-helpers' do not currently appear in command-list.txt. 'gitcredentials' was forgotten back when guides were added to command-list.txt in 1b81d8cb19 (help: use command-list.txt for the source of guides, 2018-05-20). 'gitremote-helpers' was moved to section 7 in 439cc74632 (docs: move gitremote-helpers into section 7, 2019-03-25), but command-list.txt was not updated at the time. Add these two guides to the list of guides in 'command-list.txt', so that they appear in the output of 'git help --guides', and capitalize the first word of the description of 'gitcredentials', as was done in 1b81d8c (help: use command-list.txt for the source of guides, 2018-05-20) for the other guides. While at it, add a comment in Documentation/Makefile to remind developers to update command-list.txt if they add a new guide. Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>