EventLogReader.ReadEvent does not document null-returning behaviour, IDisposable info, nor give usage example

(Creating an issue for a PR)

https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.eventing.reader.eventlogreader.readevent

None of these behaviours are documented:

- `ReadEvent()` and `ReadEvent(TimeSpan)` will return `null`...
- on the first attempt to read past the end of the query results (if running a query)
- on the first attempt to read past the last entry in the log-file on disk (if reading from a log directly).
- subsequent attempts to read will throw an `InvalidOperationException` - so reading must be stopped at the first `null`.
- The `ReadEvent` method will return `EventLogRecord` objects instead of `EventRecord` - should this be documented too?
- The `EventRecord` type is `IDisposable` because it wraps an `EventLogHandle`, but the documentation doesn't say if the `EventRecord` or `EventLogRecord` should always be disposed inside the read-loop or not, and if it's safe to pass the `EventLogRecord` around or emit it in a `yield break`.
- I assume it _should_ be eagerly-disposed and not exposed to callers, so the example in my PR copies scalar data from the `EventLogRecord` properties into a `ValueTuple` and emits those instead.
- The possible exceptions `System.Diagnostics.Eventing.Reader.EventLogException` (and subclasses) and ``InvalidOperationException` are not documented either.