rtw88: update man page for loader tunable for >4GB of main memory

Mention the loader tunable from 6a50157090f2d0c5ab8c570d9cf2e2e3535dbdbf
that needs to be set for system with more than 4GB of physical memory.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 0881eb9665dbfc8a53d9914a9dca1f0473ee1acb)