Document Charset Restriction for Html/Xml Reader

[oleibman]

Fix #1681, although probably not to the originator's satisfaction. Html and Xml readers will handle documents only with a charset of UTF-8. This PR documents that restriction. No change to source code; see the original issue for explanation.

This is:

- [ ] a bugfix
- [ ] a new feature
- [ ] refactoring
- [ ] additional unit tests
- [x] documentation update

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• 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?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.