Update "Artists with multiple names"

This proposal is an update to the "Artists with multiple names" doc[1].

• Makes greater use of artist aliases and artist credits.
• Specifically recommend avoiding the use of legal names as separate artists.
• Moves it to a section of the Style/Artists main document.
• Changes the format to a guideline format instead of a Q&A format
• Removes the style distinction between groups and individuals.

Proposal is at [2] and replaces [3].

1. http://wiki.musicbrainz.org/Style/Artist/With_multiple_names
2. http://wiki.musicbrainz.org/User:Hawke/Proposal/artists_with_multiple_names#Performance_Names
3. http://wiki.musicbrainz.org/Style/Artist