[SPARK-33641][SQL][DOC][FOLLOW-UP] Add migration guide for CHAR VARCHAR types

Author: yaooqinn

docs/sql-migration-guide.md

@@ -54,7 +54,7 @@ license: |
 
   - In Spark 3.1, creating or altering a view will capture runtime SQL configs and store them as view properties. These configs will be applied during the parsing and analysis phases of the view resolution. To restore the behavior before Spark 3.1, you can set `spark.sql.legacy.useCurrentConfigsForView` to `true`.
 
-  - In Spark 3.1, we support CHAR/CHARACTER and VARCHAR types in our type system framework instead of replacing them with STRING types. By default, they can only be used in table schema, not functions/operators. To restore the behavior before Spark 3.1, which treats them as STRING types and ignores a length parameter, e.g. `CHAR(4)`, you can set `spark.sql.legacy.charVarcharAsString` to `true`.
+  - In Spark 3.1, we support CHAR/CHARACTER and VARCHAR types in our type system framework instead of replacing them with STRING types. Currently, they can only be used in a table schema, not functions/operators. To restore the behavior before Spark 3.1, which treats them as STRING types and ignores a length parameter, e.g. `CHAR(4)`, you can set `spark.sql.legacy.charVarcharAsString` to `true`.
 
 ## Upgrading from Spark SQL 3.0 to 3.0.1