Mention "hw" (hardware flow control) in abbreviations legend

[Hedda]

Mention "hw" (hardware flow control) in abbreviations legend:

• nsw = no flow control
• sw = software flow control (XON/XOFF flow control)
• hw = hardware flow control (RTS/CTS flow control)

Hardware flow control is recommended by OpenHAB developers if firmware and dongle supports it.

Hardware flow control is however not yet supported by Home Assistant ZHA yet but it will be soon.