kern/50474: the internal mutex(9) types are undocumented

[dholland%netbsd.org@localhost]

>Number:         50474
>Category:       kern
>Synopsis:       the internal mutex(9) types are undocumented
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Nov 26 01:55:00 +0000 2015
>Originator:     David A. Holland
>Release:        NetBSD 7.99.21 (20151125)
>Organization:
>Environment:
System: NetBSD valkyrie 7.99.1 NetBSD 7.99.1 (VALKYRIE) #17: Wed Oct 14 03:21:03 EDT 2015  dholland@valkyrie:/usr/src/sys/arch/amd64/compile/VALKYRIE amd64
Architecture: x86_64
Machine: amd64
>Description:

mutex(9) says under mutex_init():

           The type argument must be given as MUTEX_DEFAULT.  Other constants
           are defined but are for low-level system use and are not an
           endorsed, stable part of the interface.

This is all very well but not much use if one is trying to work on or
understand the implementation internals. Private material should be
labeled as such, but should also still be documented.

>How-To-Repeat:

>Fix: