Updated Sc2004 (markdown)

Sc2004.md

@@ -10,6 +10,12 @@
 
 ### Rationale:
 
-The `$` on regular variables in arithmetic contexts has no purpose, and is just noise. 
+The `$` on regular variables in arithmetic contexts is unnecessary, and can even lead to subtle bugs. This is because the contents of `$((..))` is first expanded into a string, and then evaluated as an expression:
 
-The `$` does matter for special variables like `$1` vs `1`, `$#` vs `#`. ShellCheck does not produce this style suggestion for these cases. 
+    $ a='1+1'
+    $ echo $(($a * 5))    # becomes 1+1*5
+    6
+    $ echo $((a * 5))     # evaluates as (1+1)*5
+    10
+
+The `$` is unavoidable for special variables like `$1` vs `1`, `$#` vs `#`. ShellCheck does not warn about these cases.