[apm]: Document sampling.tail.discard_on_write_failure config

docs/en/observability/apm/configure/sampling.asciidoc

@@ -67,7 +67,7 @@ This final policy is used to catch remaining trace events that don't match a str
 === Storage limit
 The amount of storage space allocated for trace events matching tail sampling policies. Caution: Setting this limit higher than the allowed space may cause APM Server to become unhealthy.
 
-If the configured storage limit is insufficient, it logs "configured storage limit reached". The event will bypass sampling and will always be indexed when storage limit is reached.
+If the configured storage limit is insufficient, it logs "configured storage limit reached". When the storage limit is reached, the event will be indexed or discarded based on the <<sampling-tail-discard_on_write_failure-{input-type},Discard On Write Failure>> configuration.
 
 Default: `3GB`. (text)
 
@@ -76,6 +76,20 @@ Default: `3GB`. (text)
 | Fleet-managed     | `Storage limit`
 |====
 
+[float]
+[id="sampling-tail-discard_on_write_failure-{input-type}"]
+=== Discard On Write Failure
+Defines the indexing behavior when trace events fail to be written to storage (for example, when the storage limit is reached).
+When set to `false`, traces bypass sampling and are always indexed, which significantly increases the indexing load.
+When set to `true`, traces are discarded, causing data loss which can result in broken traces.
+
+Default: `false`. (bool)
+
+|====
+| APM Server binary | `sampling.tail.discard_on_write_failure`
+| Fleet-managed (version 8.19+)     | `Discard On Write Failure`
+|====
+
 // end::tbs-top[]
 
 [float]