Describe the semantics of the sgetpwnam() helper function

in the comment above it so that nobody will save pointers
returned inside "struct passwd" across the calls to the function.

libexec/ftpd/ftpd.c

@@ -934,6 +934,9 @@ sgetsave(char *s)
  * Save the result of a getpwnam.  Used for USER command, since
  * the data returned must not be clobbered by any other command
  * (e.g., globbing).
+ * NB: The data returned by sgetpwnam() will remain valid until
+ * the next call to this function.  Its difference from getpwnam()
+ * is that sgetpwnam() is known to be called from ftpd code only.
  */
 static struct passwd *
 sgetpwnam(char *name)