[riot-notifications] [RIOT-OS/RIOT] core/mutex: improve documentation (#15398)

[Marian Buschsieweke]

### Contribution description

As the title says.

### Testing procedure

```
make doc && firefox doc/doxygen/html/index.html
```

### Issues/PRs references

Need for better documentation become apparent in https://github.com/RIOT-OS/RIOT/pull/15327
You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/15398

-- Commit Summary --

  * core/mutex: improve documentation

-- File Changes --

    M core/include/mutex.h (80)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/15398.patch
https://github.com/RIOT-OS/RIOT/pull/15398.diff

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