Improved metadata documentation.

git-svn-id: http://music21.googlecode.com/svn/trunk@3655 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Improved metadata and corpora documentation.

git-svn-id: http://music21.googlecode.com/svn/trunk@3656 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Improved formats overview documentation.

git-svn-id: http://music21.googlecode.com/svn/trunk@3657 5c16147e-a200-11de-b0cc-ab0ad15e60d0
Writing metadata overview documentation.

git-svn-id: http://music21.googlecode.com/svn/trunk@3658 5c16147e-a200-11de-b0cc-ab0ad15e60d0