Allow prototype to be shown with currenttag

doc/tagbar.txt

@@ -611,6 +611,9 @@ tagbar#currenttag({format}, {default} [, {flags}])
     's'   If the tag is a function, the complete signature will be shown,
           otherwise just "()" will be appended to distinguish functions from
           other tags.
+    'p'   Display the raw prototype instead of the parsed tag. This can be
+          useful in cases where ctags doesn't report some information, like
+          the signature. Note that this can get quite long.
 
     For example, if you put the following into your statusline: >
         %{tagbar#currenttag('[%s] ','')}