Now explicitly defines variant occurrences of type parameters.

The discussion about SDK issue 31305 showed that the definition of
covariant/contravariant/invariant occurrences need to be given
somewhere, and also that covariant-from-class.md is a reasonable place
to put it.

This CL adds those definitions there, based on Leaf's proposal in the
above-mentioned issue, and adjusts the definition of what it means to
be a covariant parameter such that the case where the relevant type
parameter occurs in the bound of a formal type parameter in a function
type is taken into account.

(It also reformats the document to stay within 80 columns and follow
the style of newer informal specs more closely, but that should be
easy to skip over because Gerrit colors white space changes
differently from "real" changes).

Change-Id: I0b0a688c616d0bb56755ceea08e1792abfa7936d
Reviewed-on: https://dart-review.googlesource.com/23422
Commit-Queue: Erik Ernst <[email protected]>
Reviewed-by: Lasse R.H. Nielsen <[email protected]>

Author: eernstg