Enhance omitframepointer.md documentation

[PerikiyoXD]

What does this PR do?
Updates the documentation for the omitframepointer API, replacing placeholder text with clear explanations of each parameter (Default, On, Off). Also adds practical usage examples for common scenarios (debug vs release builds).

How does this PR change Premake's behavior?
This PR does not change Premake’s behavior. It only improves the generated documentation so users understand what the omitframepointer setting does.

Are there any breaking changes? Will any existing behavior change?
No breaking changes. This is a documentation-only update.

Anything else we should know?
This update makes the documentation more approachable for users who aren’t familiar with compiler frame pointer options. It explains the trade-off between performance optimizations and debugging convenience.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass (not applicable — documentation only)
• Mention any [related issues](https://github.com/premake/premake-core/issues) (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our [coding conventions](https://github.com/premake/premake-core/blob/master/CONTRIBUTING.md#coding-conventions)
• Minimize the number of commits
• Align [documentation](https://github.com/premake/premake-core/tree/master/website) to your changes

[nickclark2016]

I would suggest against direct discussion of performance and behavior changes other than the fact that the frame pointer is omitted. Instead, link out to documentation from various toolchain vendors discussing the performance and other characteristics.

Other than that, this looks good. Thank you for the contribution!

[nickclark2016]

Thanks for the contribution. In the future, please squash your branch down to a single commit.