[docs][note] Specify requirements for fuchsia graphics

Added a "Note" message explaining that the "-g" flag to run fuchsia in graphics mode can only be run via a terminal using a GUI.

Change-Id: I9f207dd835b35edd768f84071729887557fe9395

getting_started.md

@@ -138,6 +138,8 @@ found [here](https://fuchsia.googlesource.com/modular/+/HEAD/src/bootstrap/).
 
 #### Enabling Graphics
 
+Note: Graphics mode can only be run in a GUI with a terminal.
+
 To enable graphics, add the `-g` flag to `frun`:
 
 ```