Minor fixes and enhancements to phpdoc.

Change the extension of the documentation files from .txt to .md so that they can processed as markdown and displayed in github.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

Thanks to Dick Visser for writing this document.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3075 44740490-163a-0410-bde0-09ae8108e29a