MFC 309581,309582,310424: Document T6 support.

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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@310637 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f