Improve content generation prompt to reduce over-generation (#16333)

I focused on cases where we're inserting doc comments or annotations
above symbols.

I added 5 new examples to the content generation prompt, covering
various scenarios:

1. Inserting documentation for a Rust struct
2. Writing docstrings for a Python class
3. Adding comments to a TypeScript method
4. Adding a derive attribute to a Rust struct
5. Adding a decorator to a Python class

These examples demonstrate how to handle different languages and common
tasks like adding documentation, attributes, and decorators.

To improve context integration, I've made the following changes:

1. Added a `transform_context_range` that includes 3 lines before and
after the transform range
2. Introduced `rewrite_section_prefix` and `rewrite_section_suffix` to
provide more context around the section being rewritten
3. Updated the prompt template to include this additional context in a
separate code snippet

Release Notes:

- Reduced instances of over-generation when inserting docs or
annotations above a symbol.