docs: add warning

Author: kenjis

user_guide_src/source/helpers/text_helper.rst

@@ -30,6 +30,10 @@ The following functions are available:
     Generates a random string based on the type and length you specify.
     Useful for creating passwords or generating random hashes.
 
+    .. warning:: Except for type **crypto**, no cryptographically secure
+        strings are generated. Therefore, it must not be used for cryptographic
+        purposes or purposes that requires return values to be unguessable.
+
     The first parameter specifies the type of string, the second parameter
     specifies the length. The following choices are available: