Add feature and capabilities for focus reporting. Also document AX and XT even

though they aren't tmux's.

tmux.1

@@ -5556,6 +5556,10 @@ It is not normally necessary to set these manually, instead the
 .Ic terminal-features
 option should be used.
 .Bl -tag -width Ds
+.It Em \&AX
+An existing extension that tells
+.Nm
+the terminal supports default colours.
 .It Em \&Cs , Cr
 Set the cursor colour.
 The first takes a single string argument and is used to set the colour;
@@ -5568,8 +5572,19 @@ $ printf '\e033]12;red\e033\e\e'
 .Ed
 .It Em \&Cmg, \&Clmg, \&Dsmg , \&Enmg
 Set, clear, disable or enable DECSLRM margins.
+These are set automatically if the terminal reports it is
+.Em VT420
+compatible.
 .It Em \&Dsbp , \&Enbp
 Disable and enable bracketed paste.
+These are set automatically if the
+.Em XT
+capability is present.
+.It Em \&Dsfcs , \&Enfcs
+Disable and enable focus reporting.
+These are set automatically if the
+.Em XT
+capability is present.
 .It Em \&Smol
 Enable the overline attribute.
 .It Em \&Smulx
@@ -5620,6 +5635,11 @@ See the
 option above and the
 .Xr xterm 1
 man page.
+.It Em \&XT
+This is an existing extension capability that tmux uses to mean that the
+terminal supports the
+.Xr xterm 1
+title set sequences and to automatically set some of the capabilities above.
 .El
 .Sh CONTROL MODE
 .Nm