doc: usage: cmd: load: null-block-device

Document the dispatch path added by the former commit

    fs: dispatch null_dev_desc_ok filesystems before lookup

Add a null-block-device interfaces section that:

  * lists the three fstypes that can benefit of it (semihosting, ubifs,
    sandbox) and the CONFIG option that builds each
  * explains the '-' convention for the unused <dev[:part]> field

Suggested-by: Simon Glass <[email protected]>
Signed-off-by: Vincent Jardin <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

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