Add notes to rax_dns and rax_dns_record to recommend using serial: 1
Merge pull request #7501 from sivel/rax-dns-serial-doc

Add notes to rax_dns and rax_dns_record to recommend using serial: 1
Fix typo requird -> required
Merge pull request #7514 from Xerkus/patch-2

Fix typo requird -> required
Merge branch 'devel' into arbitrary_groups