msgget.2, semget.2, shmget.2: The purpose of "flags" == 0 is to obtain ID of an existing IPC object

This was implied in these pages, but the meaning of "flags" == 0
could be more explicit, as indicated by questions such as
https://stackoverflow.com/questions/49833569/flag-value-of-semget-function

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/msgget.2

@@ -50,6 +50,14 @@ system call returns the System\ V message queue identifier associated
 with the value of the
 .I key
 argument.
+It may be used either to obtain the identifier of a previously created
+message queue (when
+.I msgflg
+is zero and
+.I key
+does not have the value
+.BR IPC_PRIVATE ),
+or to create a new set.
 .PP
 A new message queue is created if
 .I key

man2/semget.2

@@ -52,6 +52,15 @@ The
 system call returns the System\ V semaphore set identifier
 associated with the argument
 .IR key .
+It may be used either to obtain the identifier of a previously created
+semaphore set (when
+.I semflg
+is zero and
+.I key
+does not have the value
+.BR IPC_PRIVATE ),
+or to create a new set.
+.PP
 A new set of
 .I nsems
 semaphores is created if

man2/shmget.2

@@ -51,6 +51,14 @@ shmget \- allocates a System V shared memory segment
 returns the identifier of the System\ V shared memory segment
 associated with the value of the argument
 .IR key .
+It may be used either to obtain the identifier of a previously created
+shared memory segment (when
+.I shmflg
+is zero and
+.I key
+does not have the value
+.BR IPC_PRIVATE ),
+or to create a new set.
 .PP
 A new shared memory segment, with size equal to the value of
 .I size