boot1.chrp: Improve comment for the SLOF hack

Since this original comment was written, the actual issue in SLOF
was diagnosed and documented in commit 424089a0fc, which fixed it
for loader(8). Update the comment in boot1 to be more informative
to future readers.

MFC after: 3 days
Reviewed by: jhibbits, adrian
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56470