Update docs with new `fx run` format

Bug: None
Test: None
Change-Id: Ifb7c85b400a48fc9913c54f57a1a17f5ed87e56a

development/workflows/testing_faq.md

@@ -71,7 +71,7 @@ while piping the output back to your workstation terminal. Slick!
 Make sure your device is running (hit Ctrl-D to boot an existing image) and
 connected to your workstation.
 If your target device is a QEMU emulator, then it needs networking to receive
-the test package. Start your emulator with `fx run -N`.
+the test package. Start your emulator with `fx run -- -N`.
 
 From your workstation, `fx run-test scenic_unittests`. The argument to
 `run-test` is the name of the test program as defined in `BUILD.gn`, for

getting_started.md

@@ -121,12 +121,21 @@ disk image:
 fx run
 ```
 
-There are various flags for `fx run` to control QEMU's configuration:
+The `fx run` command will prepare a QEMU disk image, then call `run-zircon` to
+boot Zircon. To specify the disk size, use the `-S` option:
+* `-S` sets QEMU's disk size. Units can be specified by suffix as "k", "K", "M",
+  "G", and "T".
+
+Use `fx run -h` to see all available options for `fx run`, and `fx run -- -h` to
+see all available options for `run-zircon`.
+
+There are various flags to control QEMU's configuration. As these flags must be
+forwarded to `run-zircon`, they must follow after a `--`.
 * `-m` sets QEMU's memory size in MB.
 * `-g` enables graphics (see below).
 * `-N` enables networking (see below).
 
-Use `fx run -h` to see all available options.
+Example: `fx run -S 2G -- -g`
 
 #### QEMU tips
 
@@ -141,7 +150,7 @@ support. Only the Zircon UI renders.
 To enable graphics under QEMU, add the `-g` flag to `fx run`:
 
 ```
-fx run -g
+fx run -- -g
 ```
 
 #### Enabling Network
@@ -152,7 +161,7 @@ a virtual interface for QEMU's use.
 Once this is done you can add the `-N` and `-u` flags to `fx run`:
 
 ```
-fx run -N -u $FUCHSIA_SCRIPTS_DIR/start-dhcp-server.sh
+fx run -- -N -u $FUCHSIA_SCRIPTS_DIR/start-dhcp-server.sh
 ```
 
 The `-u` flag runs a script that sets up a local DHCP server and NAT to