Add documentation for cxx

README.md

@@ -11,6 +11,10 @@ Mappings
 On the first use, define the first {motion} to exchange. On the second use,
 define the second {motion} and perform the exchange.
 
+`cxx`
+
+Like `cx`, but use the current line.
+
 `cxc`
 
 Clear any {motion} pending for exchange.

doc/exchange.txt

@@ -23,6 +23,9 @@ cx{motion}              Used in pairs.
                         Note: If a region was defined accidentally, it can
                         be cleared with |cxc|
 
+                                                *cxx* *exchange-define-line*
+cxx                     Like |cx|, but for the current line. |linewise|
+
                                                 *cxc* *exchange-clear*
 cxc                     Clear any defined region of text (previously defined
                         by a |cx| command)