vlc_waitpid: fix the documentation

It is currently unspecified what happens if the process does not exit
cleanly, so clarify that. Since this can actually happen, the
documentation ought to state what the result is to some extent.

include/vlc_spawn.h

@@ -84,7 +84,9 @@ int vlc_spawnp(pid_t *pid, const char *path, const int *fdv,
  *
  * \param pid process identifier as returned by vlc_spawn() or vlc_spawnp()
  *
- * \return This function returns the process exit code.
+ * \return If the process terminates cleanly, this function returns the exit
+ * code of the process. Otherwise, it returns an implementation-defined value
+ * that is not a valid exit code.
  */
 VLC_API
 int vlc_waitpid(pid_t pid);