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.
add module UDA grammar
Merge pull request #624 from quickfur/assert_doc

Improve assert expression documentation.
Base classes / interfaces may be template instantiations too.
Update grammar.dd.
Alias SuperClass and Interface to BasicType.

Per suggestion by Kenji.
Merge pull request #655 from quickfur/issue13448

Base classes / interfaces may be template instantiations too.
Merge pull request #665 from IgorStepanov/module-attributes

add grammar for module UDA