[ticket/11309] phpbb_extension_interface::disable_step correct docblock.

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
Merge PR #1177 branch 'erikfrerejean/ticket/11309' into develop

# By Erik Frèrejean
# Via Erik Frèrejean
* erikfrerejean/ticket/11309:
  [ticket/11309] phpbb_extension_interface::disable_step correct docblock.