docs: update vertex AI generative model documentation

[teslakoile]

This PR updates the docstrings for the TextEmbeddingModelGetEmbeddingsOperator and GenerativeModelGenerateContentOperator to reflect the latest terminology for Google's Vertex AI services.

The previous documentation referred to the "Vertex AI PaLM API," which is an outdated term. These changes replace it with more accurate descriptions, clarifying that the operators support the broader set of text and multimodal models available through the Vertex AI Generative Model API.

[boring-cyborg]

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[potiuk]

spelling issues to fix

[teslakoile]

The GitHub spell check is flagging “multimodal” as a typo, but it seems like a widely accepted term in modern AI contexts.
Should I update the spelling, or is it better to add it to the dictionary?

[Lee-W]

The GitHub spell check is flagging “multimodal” as a typo, but it seems like a widely accepted term in modern AI contexts. Should I update the spelling, or is it better to add it to the dictionary?

let's add it to the spellcheck dict and see how others think 🙂

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.