Fixed some out-of-date comments and details in audiality2.h

Fixed incorrect documentation for a2_HeaderVersion().

Added documentation for a2_LinkedVersion().

Fixed a2_IncRef() (now called a2_Retain() in the a2_UnloadAll()
documentation.

Renamed a2_Read() as a2_Run(). (We're going to use a2_Read() for a
more generic stream API. We might use a2_Read() on the root voice
for running off-line context, in case a2_Run() will be removed.)

Fixed obsolete documentation for a2_Tap() and a2_Insert(). (Those
comments were based on the initial hack solution, before the
'xinsert' unit was added.)

Removed commented out proposal for a new property API. We're not
going to do it that way.