Android: Update android studio docs

docs/android_studio.md

 # Android Studio
 
 [TOC]
 
 ## Usage
 
 ```shell
 build/android/gradle/generate_gradle.py --output-directory out-gn/Debug
 ```
 
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 ```
 
 For first-time Android Studio users:
 
 * Avoid running the setup wizard.
     * The wizard will force you to download unwanted SDK components to `//third_party/android_tools`.
     * To skip it, select "Cancel" when it comes up.
 
 To import the project:
 
-* Use "Import Project", and select the directory containing the generated project.
+* Use "Import Project", and select the directory containing the generated project, by default `out-gn/Debug/gradle`.
 
 You need to re-run `generate_gradle.py` whenever `BUILD.gn` files change.
 
 * After regenerating, Android Studio should prompt you to "Sync". If it doesn't, use:
     * Help -> Find Action -> "Sync Project with Gradle Files"
 
 
 ## How it Works
 
 Android Studio integration works by generating `build.gradle` files based on GN
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 * Configuration instructions can be found [here](http://tools.android.com/tech-docs/configuration). One suggestions:
     * Launch it with more RAM: `STUDIO_VM_OPTIONS=-Xmx2048m /opt/android-studio-stable/bin/studio-launcher.sh`
 * If you ever need to reset it: `rm -r ~/.AndroidStudio*/`
 * Import Android style settings:
     * Help -> Find Action -> "Code Style" (settings) -> Java -> Manage -> Import
         * Select `third_party/android_platform/development/ide/intellij/codestyles/AndroidStyle.xml`
 * Turn on automatic import:
     * Help -> Find Action -> "Auto Import"
         * Tick all the boxes under "Java" and change the dropdown to "All".
+* Turn on documentation on mouse hover:
+    * Help -> Find Action -> "Show quick documentation on mouse move"
+* Turn on line numbers:
+    * Help -> Find Action -> "Show line numbers"
 
 ### Useful Shortcuts
 
 * `Shift - Shift`: Search to open file or perform IDE action
 * `Ctrl + N`: Jump to class
 * `Ctrl + Shift + T`: Jump to test
 * `Ctrl + Shift + N`: Jump to file
 * `Ctrl + F12`: Jump to method
 * `Ctrl + G`: Jump to line
 * `Shift + F6`: Rename variable
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 * Basic Java editing and compiling works.
 
 ### What doesn't work (yet) ([crbug](https://bugs.chromium.org/p/chromium/issues/detail?id=620034))
 
 * Better support for instrumentation tests (they are treated as non-test .apks right now)
 * Make gradle aware of resources and assets
 * Make gradle aware of native code via pointing it at the location of our .so
 * Add a mode in which gradle is responsible for generating `R.java`
 * Add support for native code editing
 * Make the "Make Project" button work correctly