app_amd: Correct documentation to reflect functionality

Update documentation to reflect that maximum_number_of_words
has functionality inconsistent with the variable name (and inconsistent
with prior documentation.)

Update documentation for silence_threshold, which previously implied
that it was measuring time, rather than noise averages in the sample.

Update the comments in amd.conf.sample.

ASTERISK-25639 #close
Change-Id: I4b1451e5dc9cb3cb06d59b6ab872f5275ba79093