Update devsite docs

Author: dlarocque

docs-devsite/functions.functionserror.md

@@ -10,7 +10,7 @@ https://github.com/firebase/firebase-js-sdk
 {% endcomment %}
 
 # FunctionsError class
-An explicit error that can be thrown from a handler to send an error to the client that called the function.
+An error returned by the Firebase Functions client SDK.
 
 See [FunctionsErrorCode](./functions.md#functionserrorcode) for full documentation of codes.
 

docs-devsite/functions.md

@@ -27,7 +27,7 @@ Cloud Functions for Firebase
 
 |  Class | Description |
 |  --- | --- |
-|  [FunctionsError](./functions.functionserror.md#functionserror_class) | An explicit error that can be thrown from a handler to send an error to the client that called the function.<!-- -->See [FunctionsErrorCode](./functions.md#functionserrorcode) for full documentation of codes. |
+|  [FunctionsError](./functions.functionserror.md#functionserror_class) | An error returned by the Firebase Functions client SDK.<!-- -->See [FunctionsErrorCode](./functions.md#functionserrorcode) for full documentation of codes. |
 
 ## Interfaces