Changes to Dafny Reference Manual based on Rustan notes

[richardlford]

These changes are responses to almost all of Rustan's most
recent comments on the draft Dafny reference manual.
Yet to come is a re-ordering of the sections on
Expressions, and writing the section on module refinement.

[richardlford]

@RustanLeino, please take a look.

[richardlford]

I have signed the CLA Now.

[msftclas]

@richardlford, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, MSBOT;