chflags.1: Document the new UF_DONTCACHE flag

Commit 74654ba3b1 added and new chflags(1) flag called
"udontcache" or "dontcache".  This patch documents this flag.

This is a content change.

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D58181

bin/chflags/chflags.1

@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd June 12, 2018
+.Dd July 9, 2026
 .Dt CHFLAGS 1
 .Os
 .Sh NAME
@@ -123,6 +123,8 @@ set the user append-only flag (owner or super-user only)
 set the archive flag (owner or super-user only)
 .It Cm uchg , uchange , uimmutable
 set the user immutable flag (owner or super-user only)
+.It Cm udontcache , dontcache
+set the dont cache flag (owner or super-user only)
 .It Cm uhidden , hidden
 set the hidden file attribute (owner or super-user only)
 .It Cm uoffline , offline