docs(genai): clarify limitation with automatic function calling

[rite7sh]

Description

This PR documents the current behavior of the Google GenAI instrumentation
when automatic function calling is enabled.

At present, the instrumentation creates a span only for the top-level
generate_content SDK call. Internal model or tool calls triggered
automatically by the Google GenAI SDK occur within SDK internals and are
not traced as separate spans.

This clarification helps set correct expectations for users interpreting
traces and avoids confusion when multiple internal calls are executed.

Refs #4015

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Documentation change only (no runtime behavior changes)

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[rite7sh]

This is a documentation-only change and does not require a changelog entry.
Could a maintainer please add the Skip Changelog label?