Android docs: mention x86_64 emulators

Android Studio supports x86_64 emulators in addition to 32-bit x86
emulators. This mentions Android Studio's support, and why someone may
still prefer an x86 emulator. This also advises how to set GN args for
x86_64 devices.

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I6df4207419a58a8c3df8e1c237458291e4f05959
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538697
Reviewed-by: Andrew Grieve <[email protected]>
Commit-Queue: Andrew Grieve <[email protected]>
Cr-Commit-Position: refs/heads/master@{#644975}
diff --git a/docs/android_emulator.md b/docs/android_emulator.md
index 28fb84bb..68fd764 100644
--- a/docs/android_emulator.md
+++ b/docs/android_emulator.md
@@ -1,11 +1,11 @@
 # Using an Android Emulator
-Always use x86 emulators. Although arm emulators exist, they are so slow that
-they are not worth your time.
+Always use x86 emulators (or x86\_64 for testing 64-bit APKs). Although arm
+emulators exist, they are so slow that they are not worth your time.
 
 ## Building for Emulation
 You need to target the correct architecture via GN args:
 ```gn
-target_cpu = "x86"
+target_cpu = "x86"  # or "x64" if you have an x86_64 emulator
 ```
 
 ## Creating an Emulator Image