Document check-config.h and *adjust*.h as internal headers

Including *adjust*.h directly is likely to cause them to be applied at the
wrong time, resulting in an invalid or unintended configuration.

Including check_config.h at the wrong time is likely to cause spurious
errors.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/psa/crypto_adjust_config_synonyms.h b/include/psa/crypto_adjust_config_synonyms.h
index 332b622..ad71050 100644
--- a/include/psa/crypto_adjust_config_synonyms.h
+++ b/include/psa/crypto_adjust_config_synonyms.h
@@ -2,6 +2,8 @@
  * \file psa/crypto_adjust_config_synonyms.h
  * \brief Adjust PSA configuration: enable quasi-synonyms
  *
+ * This is an internal header. Do not include it directly.
+ *
  * When two features require almost the same code, we automatically enable
  * both when either one is requested, to reduce the combinatorics of
  * possible configurations.