Improve wording for ctrlp_working_path_mode

[d11wtq]

Fixes #397

From kien#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.

[d11wtq]

LGTM. The new wording is less ambiguous.

[mattn]

LGTM