Avoids:
ERROR 1170 (42000): BLOB/TEXT column 'uid' used in key specification without a key length

Update the documentation for the sqlauth module, discouraging plain text passwords and adding the appropriate security considerations.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2279 44740490-163a-0410-bde0-09ae8108e29a