Clarify some comments around SharedRecoveryState in xlog.c

SharedRecoveryState has been switched from a boolean to an enum as of
commit 4e87c48, but some comments still referred to it as a boolean.

Author: Amul Sul
Reviewed-by: Dilip Kumar, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CAAJ_b97Hf+1SXnm8jySpO+Fhm+-VKFAAce1T_cupUYtnE3Nxig

Author: michaelpq

src/backend/access/transam/xlog.c

@@ -7998,17 +7998,16 @@ StartupXLOG(void)
 	 * All done with end-of-recovery actions.
 	 *
 	 * Now allow backends to write WAL and update the control file status in
-	 * consequence.  The boolean flag allowing backends to write WAL is
-	 * updated while holding ControlFileLock to prevent other backends to look
-	 * at an inconsistent state of the control file in shared memory.  There
-	 * is still a small window during which backends can write WAL and the
-	 * control file is still referring to a system not in DB_IN_PRODUCTION
+	 * consequence.  SharedRecoveryState, that controls if backends can write
+	 * WAL, is updated while holding ControlFileLock to prevent other backends
+	 * to look at an inconsistent state of the control file in shared memory.
+	 * There is still a small window during which backends can write WAL and
+	 * the control file is still referring to a system not in DB_IN_PRODUCTION
 	 * state while looking at the on-disk control file.
 	 *
-	 * Also, although the boolean flag to allow WAL is probably atomic in
-	 * itself, we use the info_lck here to ensure that there are no race
-	 * conditions concerning visibility of other recent updates to shared
-	 * memory.
+	 * Also, we use info_lck to update SharedRecoveryState to ensure that
+	 * there are no race conditions concerning visibility of other recent
+	 * updates to shared memory.
 	 */
 	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
 	ControlFile->state = DB_IN_PRODUCTION;