Add explanation for FIPS 203 encaps and decaps input validation

[dkostic]

Issues:

CryptoAlg-2620

Description of changes:

Add explanation for FIPS 203 encaps and decaps input validation.
In particular, explain why we do only part of the checks in the ML-KEM
code itself.

Call-outs:

Point out areas that need special attention or support during the review process. Discuss architecture or design changes.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.47%. Comparing base (c2846eb) to head (b4d1863).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1884      +/-   ##
==========================================
- Coverage   78.47%   78.47%   -0.01%     
==========================================
  Files         585      585              
  Lines       99516    99516              
  Branches    14240    14243       +3     
==========================================
- Hits        78093    78091       -2     
- Misses      20787    20790       +3     
+ Partials      636      635       -1     

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