various: Add description for CI_SCRIPTS_BRANCH param

Disclose the fact that it accept Gerrit patch refspec.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ie5ad06db3dd08f77c44e19861e63067199ef8eb7
diff --git a/tf-m-build-config.yaml b/tf-m-build-config.yaml
index 1c536d9..62f8b4c 100644
--- a/tf-m-build-config.yaml
+++ b/tf-m-build-config.yaml
@@ -95,6 +95,8 @@
       - string:
           name: 'CI_SCRIPTS_BRANCH'
           default: 'master'
+          description: |
+            Branch of CI_SCRIPTS_REPO to use; also accepts Gerrit patch refspec like 'refs/changes/98/29798/1'
       - string:
           name: 'MBEDTLS_VERSION'
           default: ''