Document standard OTEL_* environment variables / system properties that are supported

[trask]

We can copy-paste the relevant env vars that we support, and their meanings, from https://github.com/open-telemetry/opentelemetry-specification/pull/666/files (once that PR lands we can possibly link to the spec and remove the copy-paste).

Note: there may be some discrepancy between the spec names and the names that we support currently.

Splitting this out from #342, and that issue can be more targeted at the instrumentation-specific config in io.opentelemetry.auto.config.Config.

For settings we currently support, see:

• IntervalMetricReader
• BatchSpanProcessor
• TraceConfig