docs: Clarifying messaging-mechanism.adoc.

[stoobie]

Description of the Changes

Clarifying the topic L1-L2 messaging.

PR Preview URL

https://starknet-io.github.io/starknet-docs/pr-857/documentation/architecture_and_concepts/Network_Architecture/messaging-mechanism/

Check List

• [x] Changes have been done against dev branch, and PR does not conflict
• [x] PR title follows the convention: <docs/feat/fix/chore>(optional scope): <description>, e.g: fix: minor typos in code

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