Change variable name and improve docs

autoload/neosnippet.vim

@@ -23,7 +23,7 @@ call neosnippet#util#set_default(
       \ 'g:neosnippet#enable_completed_snippet', 0,
       \ 'g:neosnippet#enable_complete_done')
 call neosnippet#util#set_default(
-      \ 'g:neosnippet#arguments_are_optional', 1)
+      \ 'g:neosnippet#enable_optional_arguments', 1)
 call neosnippet#util#set_default(
       \ 'g:neosnippet#enable_auto_clear_markers', 1)
 call neosnippet#util#set_default(

autoload/neosnippet/parser.vim

@@ -406,7 +406,7 @@ function! neosnippet#parser#_conceal_argument(arg, cnt, args) abort "{{{
   let outside = ''
   let inside = ''
   if (a:args != '')
-    if g:neosnippet#arguments_are_optional
+    if g:neosnippet#enable_optional_arguments
       let inside = ', '
     else
       let outside = ', '

doc/neosnippet.txt

@@ -217,13 +217,15 @@ g:neosnippet#enable_completed_snippet
 
 		The default value is 0.
 		
-					*g:neosnippet#arguments_are_optional*
-g:neosnippet#arguments_are_optional
-		If this variable is not 0, neosnippet expanded function
-		arguments will conceal commas.
+					*g:neosnippet#enable_optional_arguments*
+g:neosnippet#enable_optional_arguments
+		If this variable is not 0, neosnippet will conceal commas in
+		expanded arguments from completed snippets.
 		
 		Useful for languages where arguments are optional by default.
 
+		Note: For use with |g:neosnippet#enable_completed_snippet| = 1
+
 		The default value is 1.
 
 					*g:neosnippet#enable_auto_clear_markers*