Improved documentation for logging.
Improved documentation for logging.

Details:
- Recommended to use package level logger because module names are internal.
- Removed example for module-specific logger.
- Added example with basicConfig() convenience function.
- Added links to the relevant portions of the Python documentation
  (for log levels, standard handlers) because they are hard to find
  and not linked from the functions that use them.
- Explained how the format strings can be set, along with a link to
  the log record attributes that can be used for that.

Signed-off-by: Andreas Maier <38dbb04957d222ea6ce72de72413b1e55e665472@de.ibm.com>

zhmcclient/python-zhmcclient,zhmcclient/python-zhmcclient