The `@return` documentation of the `phpbb_extension_interface::disable_step`
method states incorrect that the method returns null, as it returns
false or a state.
PHPBB3-11309
The enable or purge operation of an extension could take a long time if an
expensive operation needs to be executed on a large set of data. To allow
this to succeed from a web interface with max_execution_time set in the
webserver's php configuration, subsequent requests must continue the
operation started earlier. So individual enable and purge implementations
must be able to spread their work across multiple steps.
PHPBB3-10323
