Document COPY TO limitation for control characters in text columns

COPY TO does not support control characters in text column values per
RFC 4180. This patch documents the limitation in the cqlsh reference,
including the security risks and alternative tools for data migration.

patch by Arvind Kandpal; reviewed by Brad Schoening, Stefan Miklosovic for CASSANDRA-21415

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: 2 people

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

@@ -461,6 +461,19 @@ 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` exports CSV using a restricted RFC 4180-compatible format and does not
+preserve control characters in text column values. Text columns containing control characters
+such as newlines (`\n`), carriage returns (`\r`), null bytes (`\x00`),
+or other non-printable characters cannot be reliably exported — values
+will be corrupted on re-import via `COPY FROM`. Beyond data integrity,
+non-printable characters in CSV output can pose security risks, including
+CSV injection and other forms of malicious data embedding. If your data
+contains such characters, consider using DSBulk, Spark, or
+`sstableloader` for data migration instead.
+====
+
 ==== Options for `COPY TO`
 
 `MAXREQUESTS`::