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.
Merge pull request #292 from quickfur/opdollar

Document opDollar (issue 7520)
fix Issue 9665 - Structure constant members can not be initialized if have opAssign

Describe about field initialization behavior.