Improve assert expression documentation.

Improve assert docs on design-by-contract page too.

Improve wording based on comments.

Incorporate more suggestions from comments.

Reword per comments.

Copy-edit.

Use less ambiguous wording for version(assert) and -release.
Update based on comments.
Update assert docs according to reviewer comments.
Merge pull request #624 from quickfur/assert_doc

Improve assert expression documentation.