fs_mgr: Fix misleading comment text regarding __ANDROID_RECOVERY__

This piece of comment was misleading because it only applies to
BOARD_USES_RECOVERY_AS_BOOT devices. Update the text to give a more
accurate description.

If BOARD_USES_RECOVERY_AS_BOOT is true,
* Recovery ramdisk IS boot ramdisk.
* init_first_stage is actually a symbolic link to
  init_second_stage.recovery, which links libfs_mgr.recovery.

If BOARD_USES_RECOVERY_AS_BOOT is not true,
* init_first_stage is a real binary in the generic ramdisk.
* init_first_stage links libfs_mgr.ramdisk.
* During recovery boot, the '/init' binary could be the
  init_first_stage from the generic ramdisk (A/B), or
  init_second_stage.recovery from the recovery ramdisk (non-A/B;
  standalone recovery partition).

Bug: 219811240
Test: None
Change-Id: Ib395a796f61869c13f1a5f1735ef17c224c26c8c
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index 996fa5e..82b5275 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -126,8 +126,12 @@
 
 bool fs_mgr_in_recovery() {
     // Check the existence of recovery binary instead of using the compile time
-    // macro, because first-stage-init is compiled with __ANDROID_RECOVERY__
-    // defined, albeit not in recovery. More details: system/core/init/README.md
+    // __ANDROID_RECOVERY__ macro.
+    // If BOARD_USES_RECOVERY_AS_BOOT is true, both normal and recovery boot
+    // mode would use the same init binary, which would mean during normal boot
+    // the '/init' binary is actually a symlink pointing to
+    // init_second_stage.recovery, which would be compiled with
+    // __ANDROID_RECOVERY__ defined.
     return fs_mgr_access("/system/bin/recovery");
 }