Clarify template property behavior in InnerBlocks documentation to specify prefill when empty

[sarthaknagoshe2002]

Fixes: #64801

What?

This PR updates the docs for using Template.
The insufficiency in the docs was highlighted in this issue

Why?

Updating the wording clarifies that the template is not tied solely to the insertion event but rather to any instance where InnerBlocks lacks existing content.

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[sarthaknagoshe2002]

@fabiankaegy could you please look into this PR?

[fabiankaegy]

Thanks for the update :)