Add statement on output limitations

Author: sgmiller

website/pages/docs/secrets/transform/index.mdx

@@ -189,6 +189,16 @@ valid input lengths would be between 6 and 56 characters.  This is because
 Of course, in the case of credit card numbers valid input would always be 16 
 decimal digits.
 
+#### Output Limitations
+
+After transformation and formatting by the template, the value is an encrypted
+version of the input with the format preserved.  However, the value itself may
+be *invalid* with respect to other standards.  For example the output credit card
+number may not validate (it likely won't create a valid check digit).  
+
+So one must consider when the outputs are stored whether validation in storage
+may reject them.
+
 ### Masking
 
 Masking performs replacement of matched characters on the input value with a