Improve wording for ctrlp_working_path_mode

Fixes #397

From https://github.com/kien/ctrlp.vim/pull/559:

> As stated in #397, I've also found the description of the search path flags a bit confusing, I had to resort to the code to understand it. Hopefully this will help.

Comment from @bronson:

> Much better, now I get it. Thanks!
> 
> Two further clarifications might help...
> 
> I wasn't sure what "disable this feature" meant... what feature are we disabling exactly? opening based on working path? Might be nice to say:
> 
> 0 or '' (empty string) - always use vim's cwd (:help getcwd)
> 
> Also, I thought the phrase "when starting up" meant vim startup, something that happens once and never again. "When opening", or "Every time it opens" might be clearer.