doc: document the X key

README.asciidoc

@@ -351,6 +351,7 @@ Movement
 
  * `x`: select line on which selection end lies (or next line when end lies on
         an end-of-line)
+ * `X`: similar to `x`, except the current selection is extended
  * `<a-x>`: expand selections to contain full lines (including end-of-lines)
  * `<a-X>`: trim selections to only contain full lines (not including last
             end-of-line)

doc/manpages/keys.asciidoc

@@ -137,6 +137,8 @@ is a sequence of non whitespace characters
 	select line on which selection end lies (or next line when end lies
 	on an end-of-line)
 
+*X*:: similar to *x*, except the current selection is extended
+
 *<a-x>*::
 	expand selections to contain full lines (including end-of-lines)