Added some comments for the vector regression testing

ci/regression.sh.in

@@ -384,6 +384,8 @@ vector()
     echo "begin vector tests..."
 
     make -C sim/simx
+    # REG_TESTS=1 informs the script to delete the previous binary after each vector test to save disk space
+    # Otherwise, the vector regression tests would run out of disk space eventually
     TOOLDIR=@TOOLDIR@ XLEN=@XLEN@ VLEN=256 REG_TESTS=1 ./tests/riscv/riscv-vector-tests/run-test.sh
 
     echo "vector tests done!"
@@ -483,7 +485,8 @@ start=$SECONDS
 
 for test in "${tests[@]}"; do
     if [[ "$test" != "vector" ]]; then
-        # We need to remove vector tests to save space
+        # We need to remove vector tests to save disk space for non-vector regression testing
+        # Otherwise, large regressions tests such as "Debug" will run out of disk space
         rm -rf tests/riscv/riscv-vector-tests
     fi
     $test