Updates to documentation concerning Value Binders

[MarkBaker]

Fix a couple of checks on characters at positions within a string before checking the length of the string when the offset may not exist

This is:

- [ ] a bugfix
- [ ] a new feature
- [X] documentation

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Update the documentation on Value Binders to include the case where numeric strings won't be converted to numbers, and reference the StringValueBinder

[oleibman]

Now that you are adding StringValueBinder to the docs, could you make it clear that String Value Binder also converts null and boolean to strings, and that a custom binder might be needed if that isn't the desired behavior? I got into a lengthy discussion with someone about that. I'll find that issue number later.

[oleibman]

Discussion to which I referred was in issue #1977.