[apm]: Document sampling.tail.discard_on_write_failure config

Author: isaacaflores2

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

@@ -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]