drm/amdgpu: update documentation for amdgpu_irq.c v3

Add/update function level documentation and add reference to amdgpu_irq.c
in amdgpu.rst

v2:
Added DOC comment
Added more explanations for amdgpu_hotplug_work_func
Properly formatted unused parameters
Properly formatted return values
Fixed usage of acronyms
More consistent styling

v3:
Removed duplicate "not"
Using '&' to refer to functions and types

Signed-off-by: Slava Abramov <slava.abramov@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>