Clarify QAN data sources and prepared-statement visibility for MySQL

documentation/docs/install-pmm/install-pmm-client/connect-database/mysql/mysql.md

@@ -119,6 +119,20 @@ While you can use both at the same time we recommend using only one--there is so
 
 The choice depends on the version and variant of your MySQL instance, and how much detail you want to see.
 
+!!! note "Query Analytics data sources and prepared statements"
+
+    Query Analytics (QAN) reads queries only from the slow query log written to a
+    file (`log_output=FILE`) or from Performance Schema. It cannot read the general
+    query log or a slow query log written to a table (`log_output=TABLE`).
+
+    If your application uses server-side prepared statements (common with drivers
+    such as JDBC), expect partial data on the Performance Schema source: query
+    examples appear as placeholders (`?`) instead of literal values, and EXPLAIN is
+    unavailable for them. This is how upstream MySQL exposes prepared statements to
+    Performance Schema, not a PMM limitation. The file-based slow query log records
+    executed statements with their real values, so examples and EXPLAIN are fuller
+    there.
+
 #### Data source comparison
 
 Here are the benefits and drawbacks of Slow query log and Performance Schema metrics sources: