[FEATURE] Missing documentation to SERIAL_TIOCSERGSTRUCT

[acassis]

Is your feature request related to a problem? Please describe.

According to CONFIG_SERIAL_TIOCSERGSTRUCT documentation:

            As a debug option, many serial bottom half drivers support the TIOCSERGSTRUCT
            that allows you to get the internal driver data structure.  By default, this
            IOCTL is not supported in order to reduce footprint.  But if (1) the driver
            supports the TIOCSERGSTRUCT ioctl, and (2) this option is selected, then
            support for the TIOCSERGSTRUCT will be enabled.

Except for that basic explanation, there is no Documentation/ about it and how to use it

Describe the solution you'd like

We need to add Documentation/ explaining how it can be used

Describe alternatives you've considered

N/A

Verification

• I have verified before submitting the report.