doc: document make lint-md-clean

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: #47926
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/contributing/using-internal-errors.md

@@ -85,6 +85,9 @@ for the error code should be added to the `doc/api/errors.md` file. This will
 give users a place to go to easily look up the meaning of individual error
 codes.
 
+In case `make lint` fails to detect the new error codes added into `errors.md`,
+the markdown linting cache must be cleaned with `make lint-md-clean`.
+
 ## Testing new errors
 
 When adding a new error, corresponding test(s) for the error message