Fix comment for MAX_BACKENDS.

This comment mentions that we check that the configured number of
backends does not exceed MAX_BACKENDS in RegisterBackgroundWorker()
and relevant GUC check hooks, neither of which has those checks
anymore.  To fix, adjust this comment to say that we do the check
in InitializeMaxBackends().

Oversights in commits 6bc8ef0 and 0b1fe14.

Reviewed-by: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/Z7zOEzz8lNjaU9yf%40nathan

Author: nathan-bossart

src/include/storage/procnumber.h

@@ -32,8 +32,8 @@ typedef int ProcNumber;
  * currently realistic configurations. Even if that limitation were removed,
  * we still could not a) exceed 2^23-1 because inval.c stores the ProcNumber
  * as a 3-byte signed integer, b) INT_MAX/4 because some places compute
- * 4*MaxBackends without any overflow check.  This is rechecked in the
- * relevant GUC check hooks and in RegisterBackgroundWorker().
+ * 4*MaxBackends without any overflow check.  We check that the configured
+ * number of backends does not exceed MAX_BACKENDS in InitializeMaxBackends().
  */
 #define MAX_BACKENDS_BITS		18
 #define MAX_BACKENDS			((1U << MAX_BACKENDS_BITS)-1)