Tracking issue for release notes of #129559: float types: document NaN bit pattern guarantees

[rustbot]

This issue tracks the release notes text for #129559.

• Issue is nominated for the responsible team (and T-release nomination is removed).
• Proposed text is drafted by team responsible for underlying change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text:

The section title will be de-duplicated by the release team with other release notes issues.
Prefer to use the standard titles from previous releases.
More than one section can be included if needed.

# Language
- [Document NaN bit patterns guarantees](https://github.com/rust-lang/rust/pull/129559)

Release blog section (if any, leave blank if no section is expected):