Add a clarifying comment

Signed-off-by: Elena Uziunaite <elena.uziunaite@arm.com>

scripts/all-helpers.sh

diff --git a/scripts/all-helpers.sh b/scripts/all-helpers.sh
index d20199d..9135daf 100644
--- a/scripts/all-helpers.sh
+++ b/scripts/all-helpers.sh
@@ -68,6 +68,7 @@
     fi
 
     # Enable PSA-based config (necessary to use drivers)
+    # MBEDTLS_PSA_CRYPTO_CONFIG is a legacy setting which should only be set on 3.6 LTS branches.
     if in_mbedtls_repo && in_3_6_branch; then
         scripts/config.py set MBEDTLS_PSA_CRYPTO_CONFIG
     fi