Add open! docs

Author: ryyppy

pages/docs/manual/latest/module.mdx

@@ -140,6 +140,23 @@ var p = School.getProfession(School.person1);
 
 </CodeTab>
 
+### Use `open!` to ignore shadow warnings
+
+There are situations where `open` will cause a warning due to existing identifiers (bindings, types) being redefined. Use `open!` to explicitly tell the compiler that this is desired behavior.
+
+```res
+let map = (arr, value) => {
+  value
+}
+
+// opening Js.Array2 would shadow our previously defined `map`
+// `open!` will explicitly turn off the automatic warning
+open! Js.Array2
+let arr = map([1,2,3], (a) => { a + 1})
+```
+
+**Note:** Same as with `open`, don't overuse `open!` statements if not necessary. Use (sub)modules to prevent shadowing issues.
+
 ### Extending modules
 
 Using `include` in a module statically "spreads" a module's content into a new one, thus often fulfill the role of "inheritance" or "mixin".