Makefile: clarify runtime relative gitexecdir

"git" built with RUNTIME_PREFIX flag turned on could figure out
gitexecdir and other paths as relative to "git" executable.

However, in the section specifies gitexecdir, RUNTIME_PREFIX wasn't
mentioned, thus users may wrongly assume that "git" always locates
gitexecdir as relative path to the executable.

Let's clarify that only "git" built with RUNTIME_PREFIX will locate
gitexecdir as relative path.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -504,8 +504,9 @@ GIT-VERSION-FILE: FORCE
 #   template_dir
 #   sysconfdir
 # can be specified as a relative path some/where/else;
-# this is interpreted as relative to $(prefix) and "git" at
-# runtime figures out where they are based on the path to the executable.
+# this is interpreted as relative to $(prefix) and "git" built with
+# RUNTIME_PREFIX flag will figure out (at runtime) where they are
+# based on the path to the executable.
 # Additionally, the following will be treated as relative by "git" if they
 # begin with "$(prefix)/":
 #   mandir