search: test concat substitution with negated patterns

Created by: rvantonder

Stacked on #15139.

No changes to behavior, just expanded documentation and a test for currently untested behavior after https://github.com/sourcegraph/sourcegraph/pull/15139.

The substituteConcat function has an auxiliary role in transformer trees like:

(concat "a" (not "b")) -> ("a" (not "b"))

This is important because our evaluator doesn't need to know how to evaluate this concat expression.