Show help text in completion

[pradyunsg]

What's the problem this feature will solve?
pip's completion does not show the help text for the various commands that are being used, which provide useful context when trying to fill in these details.

Describe the solution you'd like
pip's completion command be extended to show the help text for an option when suggesting it.

Alternative Solutions
None explored.

Additional context
Current Output (pip 10.0.1):

$ pip <TAB>
check       completion  config      download    freeze      hash        help
install     list        search      show        uninstall   wheel