Clarify event naming for different outcomes (#2010)

The "Severity and placement" section previously suggested using the same
event name (node.shutdown) at different severity levels to distinguish a
graceful shutdown (INFO) from a critical failure (ERROR).

This conflicts with the guidance in Event Naming and Verbs, which
recommends distinct event names for different outcomes. Updated the
example to use node.shutdown.complete (INFO) and node.shutdown.fail
(ERROR) so the guide is internally consistent.

This is an intentional semantic change, not just a wording tweak. The
guide now consistently says: different outcomes → different event names.
Severity reflects significance, but should not be the sole way to
distinguish success from failure.

Fixes #1972

Author: cijothomas

rust/otap-dataflow/docs/telemetry/events-guide.md

@@ -185,6 +185,9 @@ guide for naming:
   attributes.
 - Avoid synonyms that fragment cardinality across names (`finish` vs `complete`,
   `error` vs `fail`). Pick one verb set and stick to it.
+- Use **distinct event names** for different outcomes of the same operation
+  (e.g. `otlp.exporter.start.complete` and `otlp.exporter.start.fail`). Do not rely
+  solely on severity to distinguish success from failure.
 
 More precisely, in this project, event names SHOULD follow this pattern:
 `otelcol.<entity>[.<thing>].<verb>`
@@ -225,11 +228,10 @@ Optionally, add occurrence-specific attributes (dynamic context):
 
 When events are exported as logs, set an appropriate severity.
 
-Regarding severity, some events may be logged at different levels depending on
-their severity or impact. For example, a `node.shutdown` event may be logged at
-INFO level during a graceful shutdown, but at ERROR level if the shutdown is due
-to a critical failure. When exporting events as logs, choose the log level that
-best reflects the significance of the event.
+Regarding severity, choose the log level that best reflects the significance of
+the event. For example, `node.shutdown.complete` at INFO for a graceful
+shutdown and `node.shutdown.fail` at ERROR for a critical failure -- these are
+distinct events, not the same event at different severity levels.
 
 ## Stages