thumbnails: Clarify shared thumbnail URIs

Merged Kenny Levinsen requested to merge kennylevinsen/xdg-specs:shared_thumbnails into master

The thumbnail spec states in many places that URIs are always absolute, which contradicts the shared thumbnail section. The shared thumbnail section also used confusing wording to describe the way the relative URI worked, and failed to address Thumb::URI at all.

To fix this, all mentions of absolute URI are updated to state that absolute URIs are for global thumbnail repositories, while shared thumbnail repositories use URI relative to the thumbnail repository. The wording for how relative URI works in the shared thumbnail section is updated to be more explicit and to include an example, and explicit mention of Thumb::URI is added to the section as well.

This PR also includes RFC link fixup, as the old RFC links were dead.

Fixes #13 (closed).

Edited by Kenny Levinsen

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