binding: allow to rebind the word-completion function, and document it

doc/man/nanorc.5

@@ -489,6 +489,10 @@ Unindents (shifts to the left) the currently marked text.
 Comments or uncomments the current line or marked lines, using the comment
 style specified in the active syntax.
 .TP
+.B complete
+Completes the fragment before the cursor to a full word found elsewhere
+in the current buffer.
+.TP
 .B left
 Goes left one position (in the editor or browser).
 .TP

doc/texinfo/nano.texi

@@ -1091,6 +1091,10 @@ Unindents (shifts to the left) the currently marked text.
 Comments or uncomments the current line or marked lines, using the comment
 style specified in the active syntax.
 
+@item complete
+Completes the fragment before the cursor to a full word found elsewhere
+in the current buffer.
+
 @item left
 Goes left one position (in the editor or browser).
 

src/global.c

@@ -1456,6 +1456,10 @@ sc *strtosc(const char *input)
     else if (!strcasecmp(input, "comment"))
 	s->scfunc = do_comment;
 #endif
+#ifdef ENABLE_WORDCOMPLETION
+    else if (!strcasecmp(input, "complete"))
+	s->scfunc = complete_a_word;
+#endif
 #ifndef NANO_TINY
     else if (!strcasecmp(input, "indent"))
 	s->scfunc = do_indent_void;