docs: Get module_init() docs from module.h

The docs build complains:

    ./include/linux/init.h:1: warning: no structured comments found

The problem is that the comments in question were moved to module.h in
commit 0fd972a7 (module: relocate module_init from init.h to
module.h).  Fix basics.rst to match.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>