docs: show how to report all failing validators of a field at once

[totally-not-ai]

Review risk: two-way door: adds one paragraph and an inline example to the Binder validation page. No other content changes.

Summary

The Multiple Validators section of articles/flow/binding-data/components-binder-validation.adoc showed how to chain validators, but didn't say that validation stops at the first validator that fails. Users with several rules for one field, such as password rules, then only see one message at a time.

What changed

• A paragraph explaining that validators run in order, validation stops at the first failure, and the field shows only that validator's message.
• A short inline example that combines several validators into one, runs all of them, and joins the error messages, so all unmet password rules are shown at once.

This replaces the "Show more than one validation message at a time" recipe from the retired Vaadin Cookbook.

Test summary

• The snippet was compiled against the docs project's Vaadin version, but it isn't kept in the repository because it's inline.

🤖 Generated with Claude Code

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• flow/binding-data/components-binder-validation — modified, +24/-0 lines

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