docs: fix comments for request helpers

[Acconut]

The documentation comment for NewModelTextMessage and NewSystemTextMessage incorrectly referred to themselves as NewUserTextMessage.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

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