ticdc: add explanation for csv delimiter config (#15877)

Author: lidezhu

ticdc/ticdc-csv.md

@@ -23,7 +23,7 @@ protocol = "csv"
 terminator = "\n"
 
 [sink.csv]
-delimiter = ','
+delimiter = ',' # Before v7.6.0, you can only set the delimiter to a single character. Starting from v7.6.0, you can set it to 1-3 characters. For example, `$^` or `|@|`.
 quote = '"'
 null = '\N'
 include-commit-ts = true