safe_create_leading_directories(): improve docstring

Document the difference between this function and
safe_create_leading_directories_const(), and that the former restores
path before returning.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

cache.h

@@ -993,6 +993,11 @@ int adjust_shared_perm(const char *path);
  * directory while we were working.  To be robust against this kind of
  * race, callers might want to try invoking the function again when it
  * returns SCLD_VANISHED.
+ *
+ * safe_create_leading_directories() temporarily changes path while it
+ * is working but restores it before returning.
+ * safe_create_leading_directories_const() doesn't modify path, even
+ * temporarily.
  */
 enum scld_error {
 	SCLD_OK = 0,