Small documentation fixes

[mbuesch]

Some minor fixes, that slipped through before release.

• Mutex::borrow() cs parameter is not a reference (in new bare-metal).
• Mutex example: Use proper code in a function body that would actually compile.
• Document the ufmt feature.
• Cargo example version

Shall we also bump the macros edition to 2021?

[mbuesch]

Thanks. I updated the PR to include the edition change for macros.