SI-7376 Unmoored doc has correct position

The unmoored DocComment is created more eagerly so that its position
is correct despite subsequent line comments. (Previously, skipComment
would advance docPos.)

It looks like the error caret is still off by one when a doc comment
shows up in the middle of an operator, and who doesn't scaladoc the
interior of expressions?

Another bug fixed by Paul's refactor is that additional comments
between the doc and the entity no longer breaks the scaladoc.
Test added.