Clean up fastboot help:

After:

  usage: fastboot [OPTION...] COMMAND...

  flashing:
   update ZIP                 Flash all partitions from an update.zip package.
   flashall                   Flash all partitions from $ANDROID_PRODUCT_OUT.
                              On A/B devices, flashed slot is set as active.
                              Secondary images may be flashed to inactive slot.
   flash PARTITION [FILENAME]
                              Flash given partition only.

  basics:
   devices [-l]               List devices in bootloader (-l: with device paths).
   getvar NAME                Display given bootloader variable.
   reboot [bootloader]        Reboot device.

  locking/unlocking:
   flashing lock|unlock       Lock/unlock partitions for flashing
   flashing lock_critical|unlock_critical
                              Lock/unlock 'critical' bootloader partitions.
   flashing get_unlock_ability
                              Check whether unlocking is allowed (1) or not(0).

  advanced:
   erase PARTITION            Erase a flash partition.
   format[:FS_TYPE[:SIZE]] PARTITION
                              Format a flash partition.
   set_active SLOT            Set the active slot.
   oem [COMMAND...]           Execute OEM-specific command.

  boot image:
   boot KERNEL [RAMDISK [SECOND]]
                              Download and boot kernel from RAM.
   flash:raw PARTITION KERNEL [RAMDISK [SECOND]]
                              Create boot image and flash it.
   -c CMDLINE                 Override kernel command line.
   --base ADDRESS             Set kernel base address (default: 0x10000000).
   --kernel-offset            Set kernel offset (default: 0x00008000).
   --ramdisk-offset           Set ramdisk offset (default: 0x01000000).
   --tags-offset              Set tags offset (default: 0x00000100).
   --page-size BYTES          Set flash page size (default: 2048).
   --header-version VERSION   Set boot image header version.

  Android Things:
   stage IN_FILE              Sends given file to stage for the next command.
   get_staged OUT_FILE        Writes data staged by the last command to a file.

  options:
   -w                         Wipe userdata.
   -u                         Do not erase partition first when formatting.
   -s SERIAL                  Specify a USB device.
   -s tcp|udp:HOST[:PORT]     Specify a network device.
   -i VENDOR_ID               Filter devices by USB vendor id.
   -S SIZE[K|M|G]             Use sparse files above this limit (0 to disable).
   --slot SLOT                Use SLOT; 'all' for both slots, 'other' for
                              non-current slot (default: current active slot).
   --set-active[=SLOT]        Sets the active slot before rebooting.
   --skip-secondary           Don't flash secondary slots in flashall/update.
   --skip-reboot              Don't reboot device after flashing.
   --disable-verity           Sets disable-verity when flashing vbmeta.
   --disable-verification     Sets disable-verification when flashing vbmeta.
   --wipe-and-use-fbe         Enable file-based encryption, wiping userdata.
   --unbuffered               Don't buffer input or output.
   --verbose, -v              Verbose output.
   --version                  Display version.
   --help, -h                 Show this message.

Bug: N/A
Test: `fastboot --help`

Change-Id: I3639fdee68de544f12e75004ccbb03b8386e64ed