Add an I format modifier to get some bits of information about a client

(terminal features, capabilities and environment).

tmux.1

@@ -6429,6 +6429,24 @@ see
 .Xr strftime 3 .
 .Pp
 The
+.Ql I:\&
+prefix will interrogate the client.
+.Ql I/f
+will be true if
+.Nm
+believes that the client supports the given terminal feature, for example
+.Ql I/f:RGB ,
+.Ql I/c
+will be true if a client has the given
+.Xr terminfo 3
+capability, for example
+.Ql I/c:smcup ,
+and
+.Ql I/e
+gives the value of a client environment variable, for example
+.Ql #{I/e:FOO} .
+.Pp
+The
 .Ql b:\&
 and
 .Ql d:\&