logger.c: Move LOG_GROUP documentation to dedicated XML file.

The `get_documentation` awk script will only extract the first
DOCUMENTATION block that it finds in a given file. This is by design
(9bc2127d) to prevent AMI event documentation from being pulled in to
the core.xml documentation file.

Because of this, the `LOG_GROUP` documentation added in 89709e25 was
not being properly extracted and was missing fom the resulting XML
documentation file. This commit moves the `LOG_GROUP` documentation to
a separate `logger.xml` file.