Add documentation for ale-symbols feature

doc/ale.txt

@@ -432,7 +432,42 @@ vimrc, and your issues should go away. >
 
   set completeopt=menu,menuone,preview,noselect,noinsert
 <
+                                                                  *ale-symbols*
 
+ALE provides a set of basic completion symbols. If you want to replace those
+symbols with others, you can set the variable |g:ale_completion_symbols| with
+a mapping of the type of completion to the symbol or other string that you
+would like to use. An example here shows the available options for symbols  >
+
+  let g:ale_completion_symbols = {
+  \ 'text': '',
+  \ 'method': '',
+  \ 'function': '',
+  \ 'constructor': '',
+  \ 'field': '',
+  \ 'variable': '',
+  \ 'class': '',
+  \ 'interface': '',
+  \ 'module': '',
+  \ 'property': '',
+  \ 'unit': 'unit',
+  \ 'value': 'val',
+  \ 'enum': '',
+  \ 'keyword': 'keyword',
+  \ 'snippet': '',
+  \ 'color': 'color',
+  \ 'file': '',
+  \ 'reference': 'ref',
+  \ 'folder': '',
+  \ 'enum member': '',
+  \ 'constant': '',
+  \ 'struct': '',
+  \ 'event': 'event',
+  \ 'operator': '',
+  \ 'type_parameter': 'type param',
+  \ '<default>': 'v'
+  \ }
+<
 -------------------------------------------------------------------------------
 5.2 Go To Definition                                     *ale-go-to-definition*