Update README.md for logging buffering feature

[evgenyfedorov2]

1. Include the limitation that the log scope is ignored at buffer flushing. See Add logging buffering #5635 (comment)

[KalleOlaviNiemitalo]

Document that, in the Microsoft.Extensions.Telemetry implementation of BufferedLogRecord, the BufferedLogRecord.ActivitySpanId, BufferedLogRecord.ActivityTraceId, BufferedLogRecord.ManagedThreadId, and BufferedLogRecord.MessageTemplate properties are always null and thus cannot be meaningfully used by IBufferedLogger.LogRecords(IEnumerable<BufferedLogRecord>) implementations in logger provider packages. Especially document that those features cannot be enabled by setting any flags in GlobalLogBufferingOptions. Noted in #5635 (comment) and #5635 (comment).

[KalleOlaviNiemitalo]

Document that log buffers do not preserve the order of log records across categories. Noted in #5635 (comment).

[KalleOlaviNiemitalo]

Document that logging buffering applies to all logger providers, even if their loggers do not implement IBufferedLogger; and that the loss of logger scopes thus likewise applies to all logger providers. It is not possible to configure so that e.g. DebugLoggerProvider gets all log events immediately but EventLogLoggerProvider is buffered.

This is specifically a limitation of the Microsoft.Extensions.Telemetry package, rather than Microsoft.Extensions.Telemetry.Abstractions or Microsoft.Extensions.Logging.Abstractions. It is possible to implement a custom ILoggerFactory that supports per-provider configuration of buffering, like I mentioned in #5635 (comment).