Clarify publish behavior for shared_ptr and unique_ptr

[Yuchen966]

Description

Updates some explanation of intra-process communication demo document to reflect the actual current behavior with respect to function publish():

• shared_ptr messages: Cannot be published (removed incorrect zero-copy claims)
• unique_ptr messages: Zero-copy transfer achieved for the last subscriber
• raw messages: Both msg and std::move(msg) result in copying

Key changes:

• Add notes about current behaviour
• Correct the explanation that all subcribers except for the last one will receive copies of published messages.

Fixes issue

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