dmaengine: doc: fix warnings/issues of client.rst

     Note that callbacks will always be invoked from the DMA

     engines tasklet, never from interrupt context.

Optional: per descriptor metadata

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  **Optional: per descriptor metadata**

  DMAengine provides two ways for metadata support.

  DESC_METADATA_CLIENT

  DESC_METADATA_CLIENT

    - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:

      1. prepare the descriptor (dmaengine_prep_*)

         construct the metadata in the client's buffer

      2. use dmaengine_desc_attach_metadata() to attach the buffer to the

         descriptor

      3. submit the transfer

    - DMA_DEV_TO_MEM:

      1. prepare the descriptor (dmaengine_prep_*)

      2. use dmaengine_desc_attach_metadata() to attach the buffer to the

         descriptor

  DESC_METADATA_ENGINE

    - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:

      1. prepare the descriptor (dmaengine_prep_*)

      2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the

         engine's metadata area

      4. use dmaengine_desc_set_metadata_len()  to tell the DMA engine the

         amount of data the client has placed into the metadata buffer

      5. submit the transfer

    - DMA_DEV_TO_MEM:

      1. prepare the descriptor (dmaengine_prep_*)

      2. submit the transfer

      3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get

      void dma_async_issue_pending(struct dma_chan *chan);

Further APIs:

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Further APIs

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1. Terminate APIs