fix: #3593002 Clarify WebformHandlerInterface documentation

Babita Neogrequested to merge
issue/webform-3593002:3593002-provide-better-documentation into 6.3.x

Closes #3593002 Clarifies WebformHandlerInterface::access() documentation to explain that it controls access to operations on an existing webform submission.

Also updates the parameter descriptions to refer to the webform submission instead of generic entity wording.

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