MediaRecorder: Fix some audio source descriptions

voice recognition: it is always available
camcorder: "main microphone" doesn't mean anything
communication: there is no requirement for this source
  to behave like the default preset.

Test: make offline-sdk-docs
Bug: 32451872

Change-Id: I1a2f196977e547feebb031a98b0f0e560f0557d5
1 file changed