Fix Issue #2535: validation documentation lacks reset method

[bivanbi]

Description
Fixes issue #2535:
Documentation / Library Reference / Validation missed the part about reset() method and running multiple validations after one another.
Added description and emphasized significance of reset() in section Working With Validation.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs - does not apply
• Unit testing, with >80% coverage - does not apply
• User guide updated
• Conforms to style guide

[MGatner]

Thanks for this! I always like seeing the User Guide get some quality additions.

[MGatner]

@bivanbi You have some commits that were not signed; I cannot merge this until those are updated to verified commits.