Document opDollar.
Clarify about opDollar without compile-time argument.
Fix wrong example code.
Move opDollar docs to its own section.

Expand with feedback from Kenji, Don, et al.
Update LDC download links to 0.12.0.

Also rearranges the links to be in the same order as for DMD.
Merge pull request #292 from quickfur/opdollar

Document opDollar (issue 7520)
Merge pull request #402 from klickverbot/ldc-0.12.0

Update LDC download links to 0.12.0.