spec: highlight missing prose for easier review, fixed a few sections

The (temporary) highlights will make it easier to review the spec
in formatted form as opposed to html text.

Added a missing rule about the use of adjusted core types for
constraint type inference.

Adjusted rule for invalid embedding of interface types.

Change-Id: Ie573068d2307b66c937e803c486724175415b9c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/385535


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>