Fix IsUnderPostmaster/EXEC_BACKEND confusion

src/backend/utils/init/postinit.c

@@ -427,6 +427,11 @@ pg_split_opts(char **argv, int *argcp, char *optstr)
  * This must be called after modules have had the chance to register background
  * workers in shared_preload_libraries, and before shared memory size is
  * determined.
+ *
+ * Note that in EXEC_BACKEND environment, the value is passed down from
+ * postmaster to subprocesses via BackendParameters in SubPostmasterMain; only
+ * postmaster itself and processes not under postmaster control should call
+ * this.
  */
 void
 InitializeMaxBackends(void)