[SPARK-37575][SQL][FOLLOWUP] Update the migration guide for added legacy flag for the breaking change of write null value in csv to unquoted empty string

### What changes were proposed in this pull request?
This is a follow-up of updating the migration guide for #36110 which adds a legacy flag to restore the pre-change behavior.
It also fixes a typo in the previous flag description.

### Why are the changes needed?
The flag needs to be documented.

### Does this PR introduce _any_ user-facing change?
It changes the migration doc for users.

### How was this patch tested?
No tests

Closes #36268 from anchovYu/flags-null-to-csv-migration-guide.

Authored-by: Xinyi Yu <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: anchovYu

docs/sql-migration-guide.md

@@ -54,7 +54,7 @@ license: |
 
   - Since Spark 3.3, the `strfmt` in `format_string(strfmt, obj, ...)` and `printf(strfmt, obj, ...)` will no longer support to use "0$" to specify the first argument, the first argument should always reference by "1$" when use argument index to indicating the position of the argument in the argument list.
 
-  - Since Spark 3.3, nulls are written as empty strings in CSV data source by default. In Spark 3.2 or earlier, nulls were written as empty strings as quoted empty strings, `""`. To restore the previous behavior, set `nullValue` to `""`.
+  - Since Spark 3.3, nulls are written as empty strings in CSV data source by default. In Spark 3.2 or earlier, nulls were written as empty strings as quoted empty strings, `""`. To restore the previous behavior, set `nullValue` to `""`, or set the configuration `spark.sql.legacy.nullValueWrittenAsQuotedEmptyStringCsv` to `true`.
 
   - Since Spark 3.3, DESCRIBE FUNCTION fails if the function does not exist. In Spark 3.2 or earlier, DESCRIBE FUNCTION can still run and print "Function: func_name not found".
 

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -3758,7 +3758,7 @@ object SQLConf {
     buildConf("spark.sql.legacy.nullValueWrittenAsQuotedEmptyStringCsv")
       .internal()
       .doc("When set to false, nulls are written as unquoted empty strings in CSV data source. " +
-        "If set to false, it restores the legacy behavior that nulls were written as quoted " +
+        "If set to true, it restores the legacy behavior that nulls were written as quoted " +
         "empty strings, `\"\"`.")
       .version("3.3.0")
       .booleanConf