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>

Documentation/Makefile

@@ -17,6 +17,7 @@ MAN1_TXT += git.txt
 MAN1_TXT += gitk.txt
 MAN1_TXT += gitweb.txt
 
+# man5 / man7 guides (note: new guides should also be added to command-list.txt)
 MAN5_TXT += gitattributes.txt
 MAN5_TXT += githooks.txt
 MAN5_TXT += gitignore.txt

Documentation/gitcredentials.txt

@@ -3,7 +3,7 @@ gitcredentials(7)
 
 NAME
 ----
-gitcredentials - providing usernames and passwords to Git
+gitcredentials - Providing usernames and passwords to Git
 
 SYNOPSIS
 --------

command-list.txt

@@ -195,6 +195,7 @@ git-write-tree                          plumbingmanipulators
 gitattributes                           guide
 gitcli                                  guide
 gitcore-tutorial                        guide
+gitcredentials                          guide
 gitcvs-migration                        guide
 gitdiffcore                             guide
 giteveryday                             guide
@@ -204,6 +205,7 @@ githooks                                guide
 gitignore                               guide
 gitmodules                              guide
 gitnamespaces                           guide
+gitremote-helpers                       guide
 gitrepository-layout                    guide
 gitrevisions                            guide
 gitsubmodules                           guide