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