find(1): Mark -not as an extensions to POSIX

While here, change mdoc macro from Ic to Fl.

PR:		253499
Reported by:	Michael Siegel <bugcounterism at malbolge.net>

usr.bin/find/find.1

@@ -31,7 +31,7 @@
 .\"	@(#)find.1	8.7 (Berkeley) 5/9/95
 .\" $FreeBSD$
 .\"
-.Dd April 18, 2020
+.Dd February 23, 2021
 .Dt FIND 1
 .Os
 .Sh NAME
@@ -1016,9 +1016,9 @@ and
 as well as
 .Ic -amin , -anewer , -cmin , -cnewer , -delete , -empty , -fstype ,
 .Ic -iname , -inum , -iregex , -ls , -maxdepth , -mindepth , -mmin ,
-.Ic -path , -print0 , -regex , -sparse
+.Ic -not , -path , -print0 , -regex , -sparse
 and all of the
-.Ic -B*
+.Fl B*
 birthtime related primaries are extensions to
 .St -p1003.1-2001 .
 .Pp