Mention old RHEL6 defaults around max user processes.

Was made apparent that customers on RHEL6 might be hitting:
https://bugzilla.redhat.com/show_bug.cgi?id=919793

Where a seemingly old max user process constraint can impose a ludicrously low limit, which is now 4096 in RHEL >7.

While we mention nofile in:
https://backstage.forgerock.com/docs/am/5.5/install-guide/#prerequisites-file-descriptors

It might be worthwhile to point this out to users of older versions of RHEL (although I do I wonder how many people are still on RHEL6) and set a baseline of 4096 for hproc, as they might come dangerously close to the limit, making them think AM is a problem when in reality it's just a rather low default limit.