[Regex] Document the fact that expressions must have balanced parentheses.

[aercolino]

It is a requirement in Regex::count_matches(). Exceptions are currently thrown if the pattern to count contains duplicate numbers and those parentheses are not balanced. That means that all expressions should have balanced parentheses even if they contain variables, even if they are contained in variables.

I don't think it's worth to go through the pain of relaxing this requirement.

[aercolino]

Notice that the requirement for balanced parenthesis is limited to duplicate numbers subexpressions (which are alternations). In fact there is a new test (count_matches_allows_to_count_broken_expressions) which confirms that unbalanced parenthesis are supported in other places. But again, this is something to handle with care.