improve formatting for metadata

As promised over a year ago(!) on
https://github.com/mwclient/mwclient/issues/1#issuecomment-13969876

This change formats the metadata strings as italics,
and separates them from the text with a horizontal rule.
The language was also slightly reworded for increased clarity.

The result is similar to this (manually formatted) example:
https://github.com/mwclient/mwclient/issues/3