apm: change default sampling.tail.storage_limit (#581)

This PR moves the changes introduced in
elastic/observability-docs#4814 by @carsonip to
our new 9.0 docs.

Author: bmorelli25

solutions/observability/apps/transaction-sampling.md

@@ -264,9 +264,13 @@ Policies map trace events to a sample rate. Each policy must specify a sample ra
 
 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.
+A value of `0GB` (or equivalent) does not set a concrete limit, but rather allows the APM Server to align its disk usage with the disk size. APM server uses up to 80% of the disk size limit on the disk where the local tail-based sampling database is located. The last 20% of disk will not be used by APM Server. It is the recommended value as it automatically scales with the disk size.
 
-Default: `3GB`. (text)
+If this is not desired, a concrete `GB` value can be set for the maximum amount of disk used for tail-based sampling.
+
+If the configured storage limit is insufficient, it logs "configured limit reached". The event will bypass sampling and will always be indexed when storage limit is reached.
+
+Default: `0GB`. (text)
 
 |     |     |
 | --- | --- |
@@ -301,4 +305,4 @@ The service name for events to match a policy. (string)
 
 ##### **`service.environment`** [sampling-tail-service-environment-{{input-type}}]
 
-The service environment for events to match a policy. (string)
+The service environment for events to match a policy. (string)