Improve release notes tracking issue

[Urgau]

This PR improves the release notes tracking issue template by mainly

• adding headers to better distinguish the sections
• putting the choose category under the GitHub TIP section
• rewording the choose category text to simplify it
• making the main text on one line (instead of splitting it)

r? @Mark-Simulacrum

[Urgau]

Example of the modified template. (un-modified)

---

This issue tracks the release notes text for #126158.

Steps

• Proposed text is drafted by PR author (or team) making the noteworthy 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 responsible team for the underlying change should edit this section to replace the automatically generated link with a succinct description of what changed, drawing upon text proposed by the author (either in discussion or through direct editing).

# Category (e.g. Language, Compiler, Libraries, Compatibility notes, ...)
- [Disallow setting some built-in cfg via the command-line](https://github.com/rust-lang/rust/pull/126158)

Tip

Use the previous releases categories to help choose which one(s) to use.
The category will be de-duplicated with all the other ones by the release team.

More than one section can be included if needed.

Release blog section

If the change is notable enough for inclusion in the blog post, the responsible team should add content to this section.
Otherwise leave it empty.

cc {} -- origin issue/PR authors and assignees for starting to draft text