Update ADK doc according to issue #796 - 7 - Agent static_instruction parameter

[adk-bot]

This pull request updates the documentation for the static_instruction parameter of LlmAgent, as requested in issue #796.

[jcwriter74]

LlmAgent.static_instruction now supports types.ContentUnion.

The static_instruction parameter of LlmAgent now accepts a wider range of types.

Proposed Change:

In docs/agents/llm-agents.md, update the description of static_instruction.

Current state:

The documentation may not fully describe the supported types for static_instruction.

Proposed Change:

**`static_instruction` (Optional[types.ContentUnion]):** Static instruction content sent literally as system instruction.
...
**Content Support:**
Accepts `types.ContentUnion` which includes:
- `str`: Simple text instruction
- `types.Content`: Rich content object
- `types.Part`: Single part (text, inline_data, file_data, etc.)
- `PIL.Image.Image`: Image object
- `types.File`: File reference
- `list[PartUnion]`: List of parts

Reasoning:
To accurately reflect the enhanced capabilities of the static_instruction parameter.

Reference:

src/google/adk/agents/llm_agent.py

[jcwriter74]

• New PR 1888 to merge the changes in this PR 806: https://github.com/google/adk-docs/pull/806/changes

[jcwriter74]

!!! Omit last comment on @jcwriter74 jcwriter74 mentioned this pull request 8 minutes ago
#1887