Small cleanup of the merged PR

Ref #187

autoload/vimwiki/base.vim

@@ -1972,7 +1972,7 @@ function! vimwiki#base#detect_nested_syntax() "{{{
         \ 'substitute(v:val, last_word, "\\=submatch(1)", "")')
   let dict = {}
   for elem in lines
-    exe "let dict.".elem." = elem"
+    let dict[elem] = elem
   endfor
   return dict
 endfunction "}}}

doc/vimwiki.txt

@@ -2021,12 +2021,20 @@ Key                       Default value~
 automatic_nested_syntaxes  1
 
 Description~
-Allows for smaller |vimwiki-option-nested_syntaxes| dictionaries, by turning
-entry for python on the previous example unnecessary: >
-  let wiki.nested_syntaxes = {'python': 'python', 'c++': 'cpp'}
+If set, the nested syntaxes (|vimwiki-option-nested_syntaxes|) are
+automatically derived when opening a buffer.
+Just write your preformatted text in your file like this >
+ {{{xxx
+ my preformatted text
+ }}}
 
-It requires that the file is reloaded (|:edit|) after new |filetype| is
-included in a file.
+where xxx is a Vim filetype.
+
+Note that you may have to reload the file (|:edit|) to see the highlight.
+
+Since every file is scanned for the markers of preformatted text when it is
+opened, it can be slow when you have huge files. In this case, set this option
+to 0.
 
 
 *vimwiki-option-diary_rel_path*