Improve ambiguous isMACAddress() option description

The no_separators option of the isMACAddress() validator will make it only check MAC addresses without separators.
Whereas the doc is ambiguously stating that it "allows" them, leaving unclear the fact that it would reject addresses with separators.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)