Pull request: all: rm var shadowing vol. 3

Updates #2803.

Squashed commit of the following:

commit ce711ae2e08c3714a4e0c6d177c02f7801c05fce
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:58:40 2021 +0300

    all: imp code, docs

commit e91094e461893c27eda879b33b5ed2c7085510df
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:42:14 2021 +0300

    all: rm var shadowing vol. 3

HACKING.md

@@ -132,7 +132,7 @@ on GitHub and most other Markdown renderers. -->
    }
    ```
 
- *  Name the deferred errors (e.g. when closing something) `cerr`.
+ *  Name the deferred errors (e.g. when closing something) `derr`.
 
  *  No shadowing, since it can often lead to subtle bugs, especially with
     errors.