Add comment about __thread_local_inner in thread_local

[DaniPopes]

Since #108927, the __thread_local_inner macro is not declared directly after thread_local anymore, nor is the import present in the source file due to it being exported with #[macro_export].
This PR adds a comment to thread_local to help any curious reader in finding the definition of said inner macro.

[rustbot]

r? @thomcc

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[thomcc]

This feels likely to get stale to me, tbh...

[DaniPopes]

Closing due to #110861