Document start/stop_capture

[dbbnrl]

It would be helpful if someone knowledgeable could expand the documentation of Device::start_capture() and Device::stop_capture(). The existing one-liners don't mention Renderdoc and provide no real indication of what the functions are for.

Ideally this would point the user to the relevant Renderdoc documentation, briefly explain what backends/scenarios/limitations apply to usage, etc.