da.4: Don't document softtimeout

kern.cam.da.default_softtimeout currently does nothing. It sets a value
in the CCB that SIMs could look at, only none do. Leave it undocumented
until it actually does something. It was introduced to allow a 'soft
recovery' to be started when things were taking too long in the SIM by
not scheduling new I/O, or other measures the SIM knew would help when
it seemed like things were getting 'backed up'. No SIM in the FreeBSD
tree implements this and scsi_da doesn't use it other than to pass it
down, so best remove it from the documentation until it does something.

Sponsored by: Netflix
Fixes: 53af9c235f5d
Reviewed by: gbe
Differential Review: https://reviews.freebsd.org/D36518