improve accessiblity of VaAlert component

[xiongmao86]

Improve accessibility on VaAlert, and add docs for it.
Fixes #3231

I am not very sure this pr is a breaking change, is it?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

[m0ksem]

Breaking changes means that user app maybe not work after these changes. Here it is okay.

[xiongmao86]

Thanks for reviewing my pr. @m0ksem.

[m0ksem]

Thanks for reviewing my pr. @m0ksem.

Thanks for participating and making wonderful PRs!