Update GoDef help: remove explicit arguments

The GoDef command does not support specifiying identifiers on the
command line due to limitations in the Guru and Godef tools, which
operate on file positions. This change updates the help for GoDef to
remove an incorrect mention of explicit arguments.

doc/vim-go.txt

@@ -217,15 +217,14 @@ COMMANDS                                                         *go-commands*
     If [!] is not given the first error is jumped to.
 
                                                                       *:GoDef*
-:GoDef [identifier]
+:GoDef 
 gd
 CTRL-]
 
-    Goto declaration/definition for the given [identifier]. If no argument is
-    given, it will jump to the declaration under the cursor. By default the
-    CTRL-] key and the mapping `gd` are enabled to invoke :GoDef for the
+    Goto declaration/definition for the declaration under the cursor. By default
+    the CTRL-] key and the mapping `gd` are enabled to invoke :GoDef for the
     identifier under the cursor. See |'g:go_def_mapping_enabled'| to disable
-    them.
+    them.  No explicit arguments are supported.
 
     vim-go also keeps a per-window location stack, roughly analogous to how
     Vim's internal |tags| functionality works. This is pushed to every time a