Improve documentation of the emitted fields

[magnusbaeck]

The plugin documentation should state which fields are produced and what they mean. As a recent discuss.elastic.co thread reveals, the difference between the os and os.name fields isn't clear to users, and I'd say the purpose of the device field isn't obvious either.

[markwalkom]

+1

[yongtw123]

For future readers, here are the available output fields according to source code, sans the prefix:

name: the family of agent, e.g. Mobile Safari, Chrome, IE, Android (Android stock browser), ...
os: full OS name of agent, if available, e.g. Windows 7, Android 4.0.3, ...
os_name: the family of agent's OS, e.g. Windows 7 (nope the ua_parser does not distinguish Windows versions), Android, iOS, ...
os_major: major version number of agent's OS, note that for ex. "Windows 7" the value will be nil
os_minor: minor version number of agent's OS
device: full name of device from which agent has sent request, e.g. LG-XXXX, iPad, iPhone, ...
major: major version number of agent
minor: minor version number of agent
patch: patch version number of agent
build: minor patch version number of agent

I'd happier if there are fields for the full version number of agent and of agent's OS. Maybe I can fork this...