help: drop usage of 'common' and 'useful' for guides

Since 1b81d8cb19 (help: use command-list.txt for the source of guides,
2018-05-20), all man5/man7 guides listed in command-list.txt appear in
the output of 'git help -g'.

However, 'git help -g' still prefixes this list with "The common Git
guides are:", which makes one wonder if there are others!

In the same spirit, the man page for 'git help' describes the '--guides'
option as listing 'useful' guides, which is not false per se but can
also be taken to mean that there are other guides that exist but are not
useful.

Instead of 'common' and 'useful', use 'Git concept guides' in both
places. To keep the code in line with this change, rename
help.c::list_common_guides_help to list_guides_help.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-help.txt

@@ -21,8 +21,8 @@ on the standard output.
 If the option `--all` or `-a` is given, all available commands are
 printed on the standard output.
 
-If the option `--guides` or `-g` is given, a list of the useful
-Git guides is also printed on the standard output.
+If the option `--guides` or `-g` is given, a list of the
+Git concept guides is also printed on the standard output.
 
 If a command, or a guide, is given, a manual page for that command or
 guide is brought up. The 'man' program is used by default for this
@@ -58,7 +58,7 @@ OPTIONS
 
 -g::
 --guides::
-	Prints a list of useful guides on the standard output. This
+	Prints a list of the Git concept guides on the standard output. This
 	option overrides any given command or guide name.
 
 -i::

builtin/help.c

@@ -579,7 +579,7 @@ int cmd_help(int argc, const char **argv, const char *prefix)
 	}
 
 	if (show_guides)
-		list_common_guides_help();
+		list_guides_help();
 
 	if (show_all || show_guides) {
 		printf("%s\n", _(git_more_info_string));

help.c

@@ -397,10 +397,10 @@ void list_cmds_by_config(struct string_list *list)
 	}
 }
 
-void list_common_guides_help(void)
+void list_guides_help(void)
 {
 	struct category_description catdesc[] = {
-		{ CAT_guide, N_("The common Git guides are:") },
+		{ CAT_guide, N_("The Git concept guides are:") },
 		{ 0, NULL }
 	};
 	print_cmd_by_category(catdesc, NULL);

help.h

@@ -21,7 +21,7 @@ static inline void mput_char(char c, unsigned int num)
 
 void list_common_cmds_help(void);
 void list_all_cmds_help(void);
-void list_common_guides_help(void);
+void list_guides_help(void);
 
 void list_all_main_cmds(struct string_list *list);
 void list_all_other_cmds(struct string_list *list);