expand_user_path(): clarify the role of the real_home parameter

The `real_home` parameter only has an effect when expanding paths
starting with `~/`, not when expanding paths starting with `~<user>/`.
Let's make that clear.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -722,7 +722,7 @@ static struct passwd *getpw_str(const char *username, size_t len)
  * Return a string with ~ and ~user expanded via getpw*. Returns NULL on getpw
  * failure or if path is NULL.
  *
- * If real_home is true, strbuf_realpath($HOME) is used in the expansion.
+ * If real_home is true, strbuf_realpath($HOME) is used in the `~/` expansion.
  */
 char *expand_user_path(const char *path, int real_home)
 {