Document and give an example of the input size limits when using the FF3-1 transform. (#9151)

* Document and give an example of the input size limits when using the FF3-1
transform.

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

@@ -170,6 +170,35 @@ In summary, there are three ways in which the tweak value may be sourced:
 Your team and organization should weigh in on the trade-offs when it comes to
 choosing the proper tweak source to use.
 
+#### Input Limits
+
+FF3-1 specifies both minimum and maximum limits on the length of an input.  
+These limits are driven by the security goals, making sure that for a given 
+alphabet the input size does not leave the input guessable by brute force.  
+
+Given an alphabet of length A, an input length L is valid if: 
+
+* L >= 2, 
+* A^L >= 1,000,000
+* and L <= 2 * floor(log<sub>A</sub>(2^96)).
+
+As a concrete example, for handling credit card numbers, A is 10, L is 16, so
+valid input lengths would be between 6 and 56 characters.  This is because 
+10^6=1,000,000 (already greater than 2), and 2 * floor(log<sub>10</sub>(2^96)) = 56.
+
+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
@@ -221,6 +250,8 @@ The following builtin alphabets are available for use in the secret engine:
 - builtin/alphanumericupper
 - builtin/alphanumeric
 
+Custom alphabets must contain between 2 and 65536 unique characters.
+
 ## Learn
 
 Refer to the [Transform Secrets Engine](https://learn.hashicorp.com/vault/adp/transform) guide for a step-by-step tutorial.