Rollup merge of #137194 - kornelski:ftls, r=tgross35

More const {} init in thread_local

`const {}` in `thread_local!` gets an optimization just based on the syntax, rather than the expression being const-compatible. This is easy to miss, so I've added more examples to the docs.

I've also added `const {}` in a couple of places in std where this optimization has been missed.

Author: jhpratt