style.9: Add a small blurb about allowing bool

It was allowed before, but make it very explicit it is allowed now.  And
prefer 'bool' to older types that were used for the same purpose -- int and
boolean_t.

Like with the C99 fixed-width types, use common sense when changing old
code.

No igor regressions.

Suggested by:	bde <20151205031713.T3286@besplex.bde.org>
Reviewed by:	glebius, davide, bapt (earlier versions)
Reviewed by:	imp
Feedback from:	julian
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D4384

share/man/man9/style.9

@@ -26,7 +26,7 @@
 .\"	From: @(#)style	1.14 (Berkeley) 4/28/95
 .\" $FreeBSD$
 .\"
-.Dd November 22, 2015
+.Dd December 5, 2015
 .Dt STYLE 9
 .Os
 .Sh NAME
@@ -288,6 +288,30 @@ Like white-space commits, care should be taken in making
 .Vt uintXX_t
 only commits.
 .Pp
+Similarly, the project is slowly moving to use the
+.St -isoC-99
+.Vt bool
+in preference to the older
+.Vt int
+or
+.Vt boolean_t .
+New code should use
+.Vt bool ,
+and old code may be converted if it is
+reasonable to do so.
+Literal values are named
+.Dv true
+and
+.Dv false .
+These are preferred to the old spellings
+.Dv TRUE
+and
+.Dv FALSE .
+Userspace code should include
+.In stdbool.h ,
+while kernel code should include
+.In sys/types.h .
+.Pp
 Enumeration values are all uppercase.
 .Bd -literal
 enum enumtype { ONE, TWO } et;