UPDATING:  linux-c6 support notes, and how to fall back to linux-f10.

PR:		187786
MFC after:	2 days
Relnotes:	yes
Reflect the chanages in sleepqueue.h and subr_sleepqueue.c

- Priority argument is introduced to sleepq_*wait* in r177085
- sleepq_calc_signal_retval is removed from implementation
- sleepq_catch_signals is internal now

Differential Revision:	https://reviews.freebsd.org/D794
Reviewed by:	jhb
Approved by:	jhb