kconfigcheck.toml: add comment explaining CONFIG_CRYPTO_MD4 (MR 5951)

[ci:skip-build]: already built successfully in CI

kconfigcheck.toml

@@ -119,7 +119,7 @@ CRYPTO_CMAC = true
 CRYPTO_DES = true
 CRYPTO_ECB = true
 CRYPTO_HMAC = true
-CRYPTO_MD4 = true
+CRYPTO_MD4 = true  # Needed for EAP MSCHAPV2 authentication
 CRYPTO_MD5 = true
 CRYPTO_SHA1 = true
 CRYPTO_SHA256 = true