func_talkdetect.c: Fix logical errors in silence detection.
There are 3 separate changes here:
1. The documentation erroneously stated that the dsp_talking_threshold
argument was a number of milliseconds when it is actually an energy
level used by the DSP code to classify talking vs. silence.
2. Fixes a copy paste error in the argument handling code.
3. Don't erroneously switch to the talking state if we aren't actively
handling a frame we've classified as talking.
Patch inspired by one provided by Moritz Fain (License #6961).