cache.h: clarify documentation for git_path, et al

The comment above these functions actually describes
sha1_file_name, and comes from the very first revision of
git. Commit 723c31fe (Add "git_path()" and "head_ref()"
helper functions., 2005-07-05) added git_path, pushing the
comment away from the function it describes; later commits
added more functions in this block.

Let's fix the comment to describe these related functions in
more detail. Let's also make sure to point out their safer
alternatives (and move those alternatives below, which makes
more sense when reading the file).

Note that we do not need to move the existing comment to
sha1_file_name.  Commit d40d535b (sha1_file.c: document a
bunch of functions defined in the file, 2014-02-21) already
added a much more descriptive comment to it.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -708,6 +708,18 @@ extern int check_repository_format(void);
 #define DATA_CHANGED    0x0020
 #define TYPE_CHANGED    0x0040
 
+/*
+ * Return a statically allocated filename, either generically (mkpath), in
+ * the repository directory (git_path), or in a submodule's repository
+ * directory (git_path_submodule). In all cases, note that the result
+ * may be overwritten by another call to _any_ of the functions. Consider
+ * using the safer "dup" or "strbuf" formats below.
+ */
+extern const char *mkpath(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
+extern const char *git_path(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
+extern const char *git_path_submodule(const char *path, const char *fmt, ...)
+	__attribute__((format (printf, 2, 3)));
+
 extern char *mksnpath(char *buf, size_t n, const char *fmt, ...)
 	__attribute__((format (printf, 3, 4)));
 extern void strbuf_git_path(struct strbuf *sb, const char *fmt, ...)
@@ -717,11 +729,6 @@ extern char *git_pathdup(const char *fmt, ...)
 extern char *mkpathdup(const char *fmt, ...)
 	__attribute__((format (printf, 1, 2)));
 
-/* Return a statically allocated filename matching the sha1 signature */
-extern const char *mkpath(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
-extern const char *git_path(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
-extern const char *git_path_submodule(const char *path, const char *fmt, ...)
-	__attribute__((format (printf, 2, 3)));
 extern void report_linked_checkout_garbage(void);
 
 /*