fix: Clarify error level impact on sessions (#15198)

Clarify in the develop docs that only events with level error or higher
must mark sessions as errored.

Author: philipphofmann

develop-docs/sdk/telemetry/sessions/index.mdx

@@ -271,7 +271,7 @@ not report any more session updates or start a new session.
 
 SDKs are encouraged to distinguish between different ways of ending a session:
 
-- `exited`: this means the session ended cleanly. It's is the same as the 
+- `exited`: this means the session ended cleanly. It's is the same as the
   session staying in `ok` from the point of success reports. However only
   sessions ending in `exited` will be considered for session durations.
   A session is allowed to go to `exited` even if errors occurred.
@@ -314,6 +314,29 @@ count of greater than zero is considered an errored session. All crashed and
 abnormal sessions are also at all times considered errored but subtracted from
 the final errored session count.
 
+Only events that stem from `captureException`, `captureError`, or `captureEvent`
+must increment the `errors` counter when their level is `error` or higher. As SDKs
+set the default level to `error` for these methods, a level lower than `error`
+means the users changed it manually. This gives the users the ability to capture
+error events not marking sessions as errored. `captureEvent` is included because
+users might manually create exception events with it. Events stemming from
+`captureMessage` must not affect the session's error count, regardless of their
+log level. For all methods, it doesn't matter if the event has a stack trace or not.
+The following table summarizes the behavior of the methods:
+
+| method | level  | increments `errors` counter |
+| --- | --- | --- |
+| captureException / captureError | `fatal` | `yes` |
+| captureException / captureError | `error` | `yes` |
+| captureException / captureError | `warning` | `no` |
+| captureEvent  | `fatal` | `yes` |
+| captureEvent  | `error` | `yes` |
+| captureEvent  | `warning` | `no` |
+| captureMessage | `fatal` | `no` |
+| captureMessage | `error` | `no` |
+| captureMessage | `warning` | `no` |
+
+
 ### Exited
 
 A session can transition to `exited` which is exactly the same state as `ok`