drm/i915: Add gem/i915_gem_context.h to the docs

In order to prevent kernel doc warnings, also fill out docs for any
missing fields and fix those that forgot the "@".

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708154835.528166-16-jason@jlekstrand.net