termios: manual: improve the explanation of various tty concepts

It is a lot easier to understand the meaning of the tty interface if
it is explained from the beginning as conceptually emulating an RS232
serial port.  This greatly simplifies the discussions of specific
items like the meaning of line speed.

Distinguish between "modem disconnect request" (deasserting DTR) and
"modem disconnect" (DCD deasserted).  Conflating the two terms is
confusing, especially for non-RS232 devices.  In particular, on most
systems, a pseudo-terminal will *not* respond to a modem disconnect
request by triggering a modem disconnect event for the purpose of the
HUPCL flag.

It is not necessarily true that the line speed has no effect on
non-serial port devices: e.g. an SPI port may interpret it as the
clock frequency to use; however, SPI does not use asynchronous framing
bits, instead synchronization is handled by the SS# wire.  Similarly,
it is common but not by any means universal for interfaces that employ
various forms of fixed data to symbol rate encodings to encode the
data link layer bit rate rather than the physical symbol rate, which
may be higher (e.g. 8B10B) or lower (e.g. QAM/Trellis), without the
encoding or framing overhead.

Finally, a handful of devices use the line rate for entirely
nonstandard purposes.  One example is Arduino USB interfaces, which
often interprets changing the baud rate to 1200 baud as a command to
reset the device.

[ v2: removed a bogus stray chunk from editing ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

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