kern: document rationale for PRIV_REBOOT usage in kexec_load

[go4av05.pvtx_gmail.com]

Summary

Document why kexec_load() currently relies on PRIV_REBOOT for access
control, noting that loading a new kernel image is more powerful than
a traditional reboot and may warrant finer-grained privileges in the
future.

No functional change intended.

Test Plan

Not applicable (comment-only change).

[ngie]

sys/kern/kern_kexec.c

344–356

My feedback in https://reviews.freebsd.org/D54326 applies here as well.

[go4av05.pvtx_gmail.com]

Enji Cooper asked to fix * alignment.
No Functional change intended.