show_call_signatures: doc, default to 2 without 'conceal' (#994)

It basically allows to give a warning when '1' is used / set explicitly, and
the 'conceal' feature is not available.

Also defaulting to '2' (cmdline mode) then seems better than not using
it.

This is just some minor clean up before
https://github.com/davidhalter/jedi-vim/pull/652.

ftplugin/python/jedi.vim

@@ -32,7 +32,7 @@ if g:jedi#auto_initialization
         execute 'nnoremap <silent> <buffer>'.g:jedi#documentation_command.' :call jedi#show_documentation()<CR>'
     endif
 
-    if g:jedi#show_call_signatures > 0 && has('conceal')
+    if g:jedi#show_call_signatures > 0
         call jedi#configure_call_signatures()
     endif