docs: clarify HTML validator warnings for Vue-specific directives

[adityaranjan08]

What This Does

This adds a tip box to the template-syntax.md guide explaining that Vue-specific attributes like v-if, @click, and v-bind may appear as invalid in HTML validators or IDEs. These are Vue template directives and are expected to be handled by Vue's compiler.

Why

Fixes vuejs/vue#4428, which notes that new users may find these attributes invalid when writing Vue templates. This update clarifies that this is expected behavior and not a bug.

Thanks!

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