config(5): Update upper limit for maxusers on 64-bit systems

The limit of 384 maxusers for auto configuration was only imposed on
32-bit systems. Document that maxusers scales above 384 based on memory
for 64-bit systems.

PR: 204938
Reported by: David Höppner <0xffea@gmail.com>

(cherry picked from commit 191c624d9519a2767801de390b192ee7a96b41cd)