Express intent of matrix release build types explicitly

Express intent of matrix release build types explicitly

We intend to do primary testing in Jenkins with all assertions
enabled, so that we maxmize our opportunities for useful feedback. We
do so in both Debug and Release flavours, so that we have confidence
that those different builds will remain maximally useful.

Expressing that we are building the Release+assert type explicitly
will help developers understand when and why they get feedback from
Jenkins.

This also permits us to build without assertions in a convenient
way. This is useful for e.g. checking that such builds remain free of
warnings, which is a useful corner case to have changed, since a
variable or parameter might only be used in an assertion expression.
This now happens in one post-submit and all release matrix
configurations.

Updated the release-matrix contents and documentation to be more
consistent with that of the other matrices.

Noted TODO for adding icc 17 testing.

Change-Id: I774602e361163337e9602f497e2302148ccbf544