Cleaned up the flag help documentation.

The --help on master and slave have a number of issues which are
resolved with this patch:
  1. Consistency - Path, location, where are all used to describe
     the path of a directory or file.  These are cleaned up.
  2. ZK path for a file must be file:/// not file://. I get that the
     last / is part of the path, however without an example it can be confusing to newbies
  3. --registration_backoff_factor help refers to a
     register_backoff_factor value which is incorrect.
  4. Attribute's notation really needs an example (which is added).

Review: https://reviews.apache.org/r/24057