MFC 309581,309582,310424: Document T6 support.

309581:
Document support for Terminator 6 adapters in cxgbe(4) and cxgbev(4).

309582:
Bump Dd for addition of T6.

310424:
Replace passive voice with active voice and other tweaks.

• Drop uses of 'will'.
• Replace 'to use' with active voice.
• Tidy language around interrupt types and clarify that INTx doesn't work on VFs.
• Drop leading articles from sysctl/tunable descriptions.
• Tweak the wording of several sysctl/tunable descriptions.