oggenc: mention resample method in manual

[HybridDog]

https://github.com/xiph/vorbis-tools/blob/master/oggenc/man/oggenc.1#L162
How does sound become resampled if I use the --resample parameter, is the resampling mechanism optimized for the upcoming ogg compression or can I get better results if I resample the sound with sox instead of oggenc's resampling?

[rillian]

It uses a sinc-filtered resampler with a Kaiser window, based on Hanning's textbook. It's an earlier version of the speex resampler used in opusenc, firefox and a number of other tools. In my experience it's fine, and you don't need to do special resampling ahead of time.

@tdaede should vorbis-tools adopt the speex resampler implementation? Should be faster, if nothing else.