Add Printer Device Class source files to the Atmel Studio integration XML.

git-svn-id: http://lufa-lib.googlecode.com/svn/trunk@2757 d5102386-fcda-11dd-9fdb-3debd5008f28
Minor documentation improvements.

git-svn-id: http://lufa-lib.googlecode.com/svn/trunk@2758 d5102386-fcda-11dd-9fdb-3debd5008f28