[FAB-6675] Document sqlite db locked error

The "database locked" error is expected when sending
multiple parallel requests to Fabric CA server cluster
that is connected to a sqlite database. This configuration
is not supported and valid. Added a section to the
troubleshooting section of the user's guide to document
this.

Change-Id: Ic044b459d228fd4b4d5ab59de013ad833344734e
Signed-off-by: Anil Ambati <aambati@us.ibm.com>

docs/source/users-guide.rst

@@ -1479,5 +1479,13 @@ Troubleshooting
    To resolve this error, you must enroll again by repeating step 'a'.  This will issue a new ECert
    which will be stored in the current database.
 
+3. When sending multiple parallel requests to a Fabric CA Server cluster that uses shared sqlite3 databases,
+   the server occasionally returns a 'database locked' error. This is most probably because the database
+   transaction timed out while waiting for database lock (held by another cluster member) to be released.
+   This is an invalid configuration because sqlite is an embedded database, which means the Fabric CA server
+   cluster must share the same file via a shared file system, which introduces a SPoF (single point of failure),
+   which contradicts the purpose of cluster topology. The best practice is to use either Postgres or MySQL
+   databases in a cluster topology.
+
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