CentOS

The first example in the description is irrelevant – it works, but it will ignore the directory named lost+found,tmp, as requested.

However, things definitely don't work as documented, so at the very least we have a documentation bug.

The KB says:

ignore_db_dirs
Description: Comma-delimited list of directories in the data directory that are not considered as database directories. Set from --ignore-db-dir at startup.
Scope: Global
Dynamic: No
Data Type: string
Introduced: MariaDB 5.5

and

--ignore-db-dir
Commandline: --ignore-db-dir=name
Description: Tells the server that this directory can never be a database. That means two things - firstly it is ignored by the SHOW DATABASES command and INFORMATION_SCHEMA tables. And secondly, USE, CREATE DATABASE and SELECT statements will return an error if the database from the ignored list specified. Use this option several times if you need to ignore more than one directory. To make the list empty set the void value to the option as --ignore-db-dir=. Corresponding read-only variable ignore_db_dirs shows the current list.
Introduced: MariaDB 5.3.9

So, ignore_db_dirs is supposed to be a read-only variable, while ignore-db-dir is supposed to be an option. In fact, ignore-db-dirs serves as both. It can be used on server startup as an option multiple times, and at runtime it shows the combined list of provided values.
On the other hand, ignore-db_dir apparently doesn't exist at all, it only works as a prefix for ignore-db-dirs, because getopt-prefix-matching is enabled by default.

Assigning to serg to confirm that it works as designed and is a documentation issue only. I can't quite figure it out from MDEV-495.

Thanks for the correction about ignore_db_dir vs ignore_db_dirs, Elena. Edited the description to correct that misunderstanding.

Let's fix the manual to describe the current behavior.