Docs: Restructure section 9.13.

Part of restructuring work for CDD.

Test: N/A
Bug: 64811960

Change-Id: Icb98a0c74708c61cec94db74d04e17ec38ab819b
diff --git a/9_security-model/9_13_safe-mode.md b/9_security-model/9_13_safe-mode.md
index 7e246ab..c86b38e 100644
--- a/9_security-model/9_13_safe-mode.md
+++ b/9_security-model/9_13_safe-mode.md
@@ -1,22 +1,25 @@
 ## 9.13\. Safe Boot Mode
 
-Android provides a mode enabling users to boot up into a mode where only
-preinstalled system apps are allowed to run and all third-party apps are
-disabled. This mode, known as "Safe Boot Mode", provides the user the
+Android provides Safe Boot Mode, which allows users to boot up into a mode
+where only preinstalled system apps are allowed to run and all third-party
+apps are disabled. This mode, known as "Safe Boot Mode", provides the user the
 capability to uninstall potentially harmful third-party apps.
 
-Android device implementations are STRONGLY RECOMENDED to implement Safe Boot
-Mode and meet following requirements:
+Device implementations are:
 
-*  Device implementations SHOULD provide the user an option to enter Safe Boot
-   Mode from the boot menu which is reachable through a workflow that is different
-   from that of normal boot.
+*   [SR] STRONGLY RECOMMENDED to implement Safe Boot Mode.
 
-*  Device implementations MUST provide the user an option to enter Safe Boot Mode
-   in such a way that is uninterruptible from third-party apps installed on
-   the device, except for when the third party app is a Device Policy Controller
-   and has set the [`UserManager.DISALLOW_SAFE_BOOT`](https://developer.android.com/reference/android/os/UserManager.html#DISALLOW_SAFE_BOOT)
-   flag as true.
+If device implementations implement Safe Boot Mode, they:
 
-*  Device implementations MUST provide the user the capability to uninstall
-   any third-party apps within Safe Mode.
+*   [C-1-1] MUST provide the user an option to
+    enter Safe Boot Mode in such a way that is uninterruptible from third-party
+    apps installed on the device, except when the third-party app is a
+    Device Policy Controller and has set the [`UserManager.DISALLOW_SAFE_BOOT`](
+    https://developer.android.com/reference/android/os/UserManager.html#DISALLOW_SAFE_BOOT)
+    flag as true.
+
+*   [C-1-2] MUST provide the user the capability to
+    uninstall any third-party apps within Safe Mode.
+
+*   SHOULD provide the user an option to enter Safe Boot Mode from the
+boot menu using a workflow that is different from that of a normal boot.