Drop `@` documentation references from functions and external types

Drop `@` documentation references from functions and external types

`@` references are used to reference function parameters, struct members
or enum variants _within_ the current type/function.  It cannot and
should not be used to reference to types outside that.

Since C has no notion of member functions it makes little sense to
prefix these with `@`; most of the documentation here was referencing
functions on _different_ types anyway.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1090>