[Docs] Clarify return value for opus_decode() and opus_decode_float() functions

[MrOnlineCoder]

Although being explicitly mentioned in the opus_decoder group in documentation (like this https://opus-codec.org/docs/opus_api-1.5/group__opus__decoder.html#details), the actual functions documentation (@returns annotations) for opus_decode() and opus_decode_float() do not specify if the returned value is equal to amount of samples per channel, or samples of all channels combined (basically amount of elements in output pcm array). I've been integrating Opus and some other codecs into a project at once, and because of a bit different APIs, silently relied on inline functions documentation, and mistakenly assumed that returned amount of samples is both channels combined, resulting in wrong audio output and a bit of debugging :D. The fact that output PCM values are interleaved also contributed to that. So this PR just adds that small clarification to both of these functions.

Blame of not being an attentive documentation reader is still surely on me ;)