GstAdapter: Improve documentation for gst_adapter_available()
Submitted by Alicia Boya García
When I first read the documentation of gst_adapter_available() and
gst_adapter_available_free() I got quite confused as it seemed that both
performed the same purpose but one was slower than the other.
I shared it with other people and found they also arrived at the same
Hopefully this patch will make the actual purpose clearer.