GstAdapter: Improve documentation for gst_adapter_available()

Submitted by Alicia Boya García @ntrrgc

Link to original bug (#793011)

Description

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
wrong conclusion.

Hopefully this patch will make the actual purpose clearer.