Usage message for innodb_compression_algorithm system variable is missing "snappy"

The usage message for the innodb_compression_algorithm system variable is currently wrong.

https://mariadb.com/kb/en/library/innodb-system-variables/#innodb_compression_algorithm

https://mariadb.com/kb/en/library/compression/#choosing-a-page-compression-algorithm

The current usage message does not list snappy, which was added as an optional compression algorithm in MariaDB 10.1.3.

The current message looks like this:

sudo mysqld --help --verbose

...

  --innodb-compression-algorithm[=name]

                      Compression algorithm used on page compression. One of:

                      none, zlib, lz4, lzo, lzma, or bzip2

...

The usage message is defined here:

https://github.com/MariaDB/server/blob/mariadb-10.4.8/storage/innobase/handler/ha_innodb.cc#L19659

You can see that the list in the usage message is hard-coded, but it is missing snappy.

Perhaps the list could instead be built dynamically from the values in the page_compression_algorithms array:

https://github.com/MariaDB/server/blob/mariadb-10.4.8/storage/innobase/handler/ha_innodb.cc#L19653

Non-supported compression algorithms are also compiled in as available options. Instead, support for the algorithm is checked at run-time in the innodb_compression_algorithm_validate() function:

https://github.com/MariaDB/server/blob/mariadb-10.4.8/storage/innobase/handler/ha_innodb.cc#L21190