Clarify AudioEncoderConfig.sampleRate semantics

[chcunningham]

Semantics of AudioEncoderConfig.sampleRate are not that "we will encode with this sampleRate", but rather: "this is the sampleRate of our input". We may resample to a different rate where required by underlying encoder libraries. For example libopus only supports a handful of rates.

We should update the dict-member dfn to reflect this.

For concerns about resampling's affect on quality, please see
https://wiki.xiph.org/OpusFAQ#But_won.27t_the_resampler_hurt_the_quality.3F_Isn.27t_it_better_to_use_44.1_kHz_directly.3F

Aside: Chrome has a bug where AudioEncoder may emit an AudioDecoderConfig with incorrect sampleRate.
https://bugs.chromium.org/p/chromium/issues/detail?id=1254496