Fix documentation for POSIX semaphores.

The documentation for POSIX semaphores is missing a reference to
max_wal_senders.  This commit fixes that in the same way that
commit 4ebe51a fixed the same issue in the documentation for
System V semaphores.

Discussion: https://postgr.es/m/20240517164452.GA1914161%40nathanxps13
Backpatch-through: 12

Author: nathan-bossart

doc/src/sgml/runtime.sgml

@@ -884,7 +884,8 @@ psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: No such
     When using POSIX semaphores, the number of semaphores needed is the
     same as for System V, that is one semaphore per allowed connection
     (<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
-    (<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
+    (<xref linkend="guc-autovacuum-max-workers"/>), allowed WAL sender process
+    (<xref linkend="guc-max-wal-senders"/>), and allowed background
     process (<xref linkend="guc-max-worker-processes"/>).
     On the platforms where this option is preferred, there is no specific
     kernel limit on the number of POSIX semaphores.