[chore] [processor/groupbyattrs] clarify compaction occurs even when …

…no key matches (open-telemetry#31027)

I expected the processor to be a complete no-op if no attribute keys
match, but I noticed that it still does perform compaction, as if the
processor was configured with an empty config (no keys). In some sense
it feels like having both grouping and compaction features in one
processor is confusing, but it's not that bad in practice but this doc
clarification can make it less surprising.

processor/groupbyattrsprocessor/README.md

@@ -178,7 +178,7 @@ processors:
 The `keys` property describes which attribute keys will be considered for grouping:
 
 * If the processed span, log record and metric data point has at least one of the specified attributes key, it will be moved to a *Resource* with the same value for these attributes. The *Resource* will be created if none exists with the same attributes.
-* If none of the specified attributes key is present in the processed span, log record or metric data point, it remains associated to the same *Resource* (no change).
+* If none of the specified attributes key is present in the processed span, log record or metric data point, it remains associated to the same *Resource* (no change), with multiple instances of the same *Resource* still [compacted](#compaction).
 
 Please refer to: