VITIS-8730: documentation of write device memory directly

[vboggara-xilinx]

Problem solved by the commit

Added documentation of read/write device memory directly

Bug / issue (if any) fixed, which PR introduced the bug, how it was discovered

https://jira.xilinx.com/browse/VITIS-8730

How problem was solved, alternative solutions (if any) and why they were rejected

To avoid of host memcpy with the usage of xrt::bo::sync(), Vitis AI runtime (VART) currently leverages xclUnmgdPwrite() and xclUnmgdPread() to perform PCIe DMA data transfer between host and device so as to achieve better performance.

Here is how xrt::bo::read and xrt::bo::write API’s to read/write directly from/to device buffer if buffer object has not host backing storage.

xrt::bo::write(const void* src, size_t size, size_t seek): Copies data from src to device buffer directly.
xrt::bo::read(void* dst, size_t size, size_t skip): Copies data from device buffer to dst.

Risks (if any) associated the changes in the commit

low

What has been tested and how, request additional testing if necessary

n/a

Documentation impact (if any)

yes

[gbuildx]

Build Passed!

[chvamshi-xilinx]

We dont need to mention about xclUnmgtPread and xclUnmgdPwrite in our documentation.
Remove the above sentence completely.

[chvamshi-xilinx]

We dont need to mention about VART also.

[chvamshi-xilinx]

After looking at overall documentation. I feel, it is better to add this info in Section "I. Data transfer between host and device by Buffer read/write API" at the end.

[uday610]

Does this only work for device_only buffers? If yes, then please mention that explicitly with a one line of code that showing the buffer creation with the appropriate flag.

[vboggara-xilinx]

closing this pull request as latest commits are not coming to this PR. Seems some issue with the PR.