Update Agraol exception message in docs

Following #40779 / agroal/agroal@6c62f82,
the exception message changed and is a bit more explicit.

(cherry picked from commit 2d99f1d)

docs/src/main/asciidoc/datasource.adoc

@@ -531,7 +531,7 @@ would result in an exception similar to this:
 Caused by: java.sql.SQLException: Exception in association of connection to existing transaction
  at io.agroal.narayana.NarayanaTransactionIntegration.associate(NarayanaTransactionIntegration.java:130)
  ...
-Caused by: java.sql.SQLException: Unable to enlist connection to existing transaction
+Caused by: java.sql.SQLException: Failed to enlist. Check if a connection from another datasource is already enlisted to the same transaction
  at io.agroal.narayana.NarayanaTransactionIntegration.associate(NarayanaTransactionIntegration.java:121)
  ...
 ----