Add note about range operators being treated as postfix unary operators

Author: hsutter

docs/cpp2/common.md

@@ -222,6 +222,8 @@ Postfix notation lets the code read fluidly left-to-right, in the same order in
 
 > Note: The `...` pack expansion syntax is also supported.
 
+> Note: The `(` `)`, `[` `]`, `..<`, and `..=` operators are treated as postfix unary operators, though they can take additional arguments.
+
 > Note: Because `++` and `--` always have in-place update semantics, we never need to remember "use prefix `++`/`--` unless you need a copy of the old value." If you do need a copy of the old value, just take the copy before calling `++`/`--`. When you write a copyable type that overloads `operator++` or `operator--`, cppfront generates also the copy-old-value overload of that function to support natural use of the type from Cpp1 code.