add small section explaining why is recommended to use these prop names

Author: umbynos

docs/guides/secure-boot.md

@@ -54,3 +54,6 @@ The security keys can be added with:
 - `build.keys.sign_key` indicates the name of the custom signing key to use to sign a binary during the compile process.
 - `build.keys.encrypt_key` indicates the name of the custom encryption key to use to encrypt a binary during the compile
   process.
+
+It's suggested to use the property names mentioned before, because they can be overridden respectively with
+`--keys-keychain`, `--sign-key` and ``--encrypt-key` Arduino CLI [compile flags](../commands/arduino-cli_compile.md).