[doc] specify that each RtMidiIn instance may have its own thread.

Fixes #194.

Author: radarsat1

doc/doxygen/tutorial.txt

@@ -196,7 +196,7 @@ int main()
 
 \section input MIDI Input
 
-The RtMidiIn class uses an internal callback function or thread to receive incoming MIDI messages from a port or device.  These messages are then either queued and read by the user via calls to the RtMidiIn::getMessage() function or immediately passed to a user-specified callback function (which must be "registered" using the RtMidiIn::setCallback() function).  We'll provide examples of both usages.
+The RtMidiIn class uses an internal callback function or thread to receive incoming MIDI messages from a port or device.  These messages are then either queued and read by the user via calls to the RtMidiIn::getMessage() function or immediately passed to a user-specified callback function (which must be "registered" using the RtMidiIn::setCallback() function).  Note that if you have multiple instances of RtMidiIn, each may have its own thread.  We'll provide examples of both usages.
 
 The RtMidiIn class provides the RtMidiIn::ignoreTypes() function to specify that certain MIDI message types be ignored.  By default, system exclusive, timing, and active sensing messages are ignored.