Fix the creaderasync example name in the qdoc file

This change eliminates the following warnings when using "make docs":

/home/lpapp/Projects/qt/qt5/qtserialport/examples/serialport/doc/creaderasync.qdoc:28:
warning: Cannot find file 'creader/creader.pro' or 'creader/creader.qmlproject'

/home/lpapp/Projects/qt/qt5/qtserialport/examples/serialport/doc/creaderasync.qdoc:28:
warning:   EXAMPLE PATH DOES NOT EXIST: creader

It is also visible that the example is unreachable on the following
documentation page:

http://qt-project.org/doc/qt-5/examples.html

Change-Id: I09f1532fcff6b1f7bfc5c4857bd14252aceb2eb3
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Refactor the reference in the documentation for property setters/getters

The documentation contains broken links due to this, and it is also inconsistent
with the rest. Properties should be referred to as properties, not individual
methods like setter, getter, notified signal, etc.

The change is tested with "make docs" and the broken links are history.

Change-Id: Id94cba1f88503754318c865793229e3d3947bec6
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Obsolete the TxD/RxD signals

They are not much of use due to the low-level OS limitations as of now, hence
the obsolete proposal. It can be brought back to life at any point when we have
a more clear idea what to do with this API.

"faking" this signal as RealTterm probably does it, is not a nice impression
for the end users. That would be more like a software "signal" (which we
already have in place) rather than a real pinout hardware signal.

It is interesting why Windows, Linux, etc do not provide an option for this.
There must be some strong reason for it.

Change-Id: Ia065e2ee9226d16e724f5e2690b25b954329d78e
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>