CASSANDRA-21381: Document COPY TO limitation for control characters in text columns

Author: arvindKandpal-ksolves

doc/modules/cassandra/pages/managing/tools/cqlsh.adoc

@@ -461,6 +461,17 @@ value `STDOUT` (without single quotes) to print the CSV to stdout.
 See `shared-copy-options` for options that apply to both `COPY TO` and
 `COPY FROM`.
 
+[NOTE]
+====
+`COPY TO` only supports printable characters in text column values, in
+accordance with RFC 4180. Text columns containing control characters
+such as newlines (`\n`), carriage returns (`\r`), null bytes (`\x00`),
+or other non-printable characters will not round-trip cleanly — values
+will be corrupted on re-import via `COPY FROM`. If your data contains
+such characters, consider using `sstableloader` for data
+migration instead.
+====
+
 ==== Options for `COPY TO`
 
 `MAXREQUESTS`::