Docs Fix: Add in the required type for the content part

[weierophinney]

The current documentation is missing this crucial requirement for the multipart/alternative + attachments section.

This had me stumped for a long time and I finally found the solution here:
https://akrabat.com/sending-attachments-in-multipart-emails-with-zendmail/

Note that the \n character is not necessary (and I think in some clients causes errors).

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Originally posted by @internalsystemerror at zendframework/zend-mail#228

[samsonasik]

this seems already in the doc https://docs.laminas.dev/laminas-mail/message/attachments/#multipartalternative-emails-with-attachments

I am closing it.