Fixing serial buffer docs (#600)

* Fixing serial buffer docs

* updated shims

libs/core/serial.cpp

@@ -121,7 +121,7 @@ namespace serial {
     }
 
     /**
-    * Reads multiple characters from the receive buffer and fills a user buffer.
+    * Reads multiple characters from the receive buffer. Pauses until enough characters are present.
     * @param length default buffer length, eg: 64
     */
     //% blockId=serial_readbuffer block="serial|read buffer %length"