shell: document another arithmetic discrepancy with bash

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash_test/ash-arith/arith-assign-in-varexp1.right

@@ -0,0 +1,2 @@
+7:7
+x=3

shell/ash_test/ash-arith/arith-assign-in-varexp1.tests

@@ -0,0 +1,9 @@
+exec 2>&1
+a='x=1'
+b='x=2'
+c='x=3'
+# The variables should evaluate immediately when they encountered,
+# not when they go into an operation. Here, order of evaluation
+# of names to numbers should be a,b,c - not b,c,a:
+echo 7:$((a+b*c))
+echo "x=$x"