Add clarifying comment about deterministic holdout selection

Co-authored-by: muzahidul-opti <129880873+muzahidul-opti@users.noreply.github.com>

Author: Copilot

lib/optimizely/config/datafile_project_config.rb

@@ -198,6 +198,7 @@ def initialize(datafile, logger, error_handler)
         flag_id = feature_flag['id']
 
         # Prefer explicit holdouts (includedFlags) over global holdouts
+        # If multiple explicit holdouts exist, use the first one (deterministic based on datafile order)
         applicable_holdout = @included_holdouts[flag_id]&.first
 
         # Use first global holdout that doesn't exclude this flag if no explicit holdout