document auto option for __fish_git_prompt_showupstream

sphinx_doc_src/cmds/fish_git_prompt.rst

@@ -20,6 +20,8 @@ There are numerous customization options, which can be controlled with git optio
 
 - ``$__fish_git_prompt_showupstream`` can be set to a number of values to determine how changes between HEAD and upstream are shown:
 
+     ``auto``
+          summarize the difference between HEAD and its upstream
      ``verbose``
           show number of commits ahead/behind (+/-) upstream
      ``name``
@@ -112,5 +114,3 @@ A simple prompt that displays git info::
         set -g __fish_git_prompt_showupstream auto
         printf '%s %s$' $PWD (fish_git_prompt)
     end
-
-