RTEMS | doc: Document B0 baud rate handling in POSIX Users Guide (#5815)

[SHIVAM DEOLANKAR (@D3athSkulll)]

Issue created by SHIVAM DEOLANKAR: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5815



## Summary

The termios implementation does not support output baud rate B0 when configuring device attributes through `tcsetattr()`.

Document that requests to set the output baud rate to B0 fail with `ENOTSUP`, since the corresponding hangup operation is not supported by the termios device configuration interface.

Update the relevant POSIX Users Guide documentation accordingly.

This is the [URL](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/termios.h.html) for Baud Rate functions from termios.h documentation should be referenced .

## Steps to reproduce

Not Applicable

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View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5815
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