Add docs for MX, SSHFP, SPF and SRV record set types

This completes the record sets documentation. Only add docs were included
because the update, view and delete can be derived using A record set
documentation. Added a small description for the new recordset types.

Change-Id: Iab12c07342db968cf130ccd09ecb7b6adf8b5007
Implements: recordset-record-adocumentation