unsuppress the help for --log-file

the changes:

* unsuppress the help for `--log-file`
* added better help text for both `--log` and `--log-file` (they do serve different functions)
* updated the docs we have on logging (that I added a few weeks back;http://www.pip-installer.org/en/develop/logic.html#logging )

One might argue that it's confusing to have 2 options showing with a similar name, but IMO it's more confusing to hide one of them, especially given that the one that's hidden, controls the log that is active by default.

fwiw, I considered a PR awhile back that would have aliased these two, but was concerned about backward compatibility, so didn't move forward with it.