Update shims.md

Author: DeveloperTryingToCodeLikeOtherOfThem

docs/shims.md

@@ -1,3 +1,4 @@
 ## Shim
 A **shim** is a parser that accesses **only** C++ or the TypeScript Abstraction Layer (TSAL), **not the sandbox environment**, for functions.
-For instance, these shims are **not** able to access properties or anything else in the runtime, which is how they were built.
+For instance, these shims are **not** able to access properties or anything else in the runtime, which is how they were built. Additionally,
+**only** functions that are exposed in the pxsim namespace, which are exported in the [pxtsim](https://github.com/microsoft/pxt/tree/master/pxtsim/) folder.