stackdriver: Refactor special fields docs

The special fields documentation as it existed was a bit of a mishmash
of old documentation for a previous logging agent solution and a lot of
additions over the years trying to match the old format. It ends up
reproducing a lot of information that is better stated by the existing
Cloud Logging docs, while missing crucial details about how the plugin
handled these cases. Certain fields were also missing.

This PR rewrites the documentation, doing more linking out to Google
Cloud Logging documentation in place of rewording the same information.
It also guides more generally on some of the edge cases for how the
special fields are handled, calling out the exceptions in a clearer way.
It also adds some fields that were previously missing from this
document.

Signed-off-by: braydonk <[email protected]>

Author: braydonk

pipeline/outputs/stackdriver.md

@@ -55,7 +55,7 @@ If you are using a _Google Cloud Credentials File_, the following configuration
 
 Example configuration file for k8s resource type:
 
-local_resource_id is used by stackdriver output plugin to set the labels field for different k8s resource types. Stackdriver plugin will try to find the local_resource_id field in the log entry. If there is no field logging.googleapis.com/local_resource_id in the log, the plugin will then construct it by using the tag value of the log.
+`local_resource_id` is used by stackdriver output plugin to set the labels field for different k8s resource types. Stackdriver plugin will try to find the local_resource_id field in the log entry. If there is no field logging.googleapis.com/local_resource_id in the log, the plugin will then construct it by using the tag value of the log.
 
 The local_resource_id should be in format: