mac/arm: Update docs -- everything builds and runs now!

Startup is pretty slow at the moment for reasons not yet known,
but other than that things seem to work.

We haven't tried running any of the test binaries yet.

Bug: 1098899
Change-Id: I5d7090de4769821d2536334134be31b518de5bc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293166
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787515}

docs/mac_arm64.md

@@ -7,12 +7,17 @@ This document describes the state of native binaries for these Macs.
 Building _for_ arm Macs
 -----------------------
 
-We're currently bringing up the build. At the moment, not everything compiles.
+We're currently bringing up the build. At the moment, everything compiles
+in a vanilla release build (we haven't tried anything else) and Chromium
+starts up fine and seems to work. We haven't tried running tests yet.
+
 You can follow the [tracking bug](https://crbug.com/1098899) to get updates on
 progress.
 
 There's a [bot](https://ci.chromium.org/p/chromium/builders/ci/mac-arm64) that
-builds for arm that you can look at to get an idea of the current state.
+builds for arm that you can look at to get an idea of the current state. It
+cross-builds on an Intel machine, and we don't have enough hardware to
+continuously run tests.
 
 To build for arm64, you have to do 2 things:
 
@@ -23,8 +28,8 @@ To build for arm64, you have to do 2 things:
        "custom_vars": { "mac_xcode_version": "xcode_12_beta" },
 
    Then just `gclient sync` and you'll automatically get that SDK and will build
-   against it. Else, manually download and install Xcode 12 beta and make it the
-   active xcode with `xcode-select`.
+   against it. Else, manually download and install Xcode 12 for macOS Universal
+   Apps Beta 2 and make it the active xcode with `xcode-select`.
 
 2. Add `target_cpu = "arm64"` to your `args.gn`.