An indexical expression is one whose meaning depends on context of utterance: "here," "now," "I," "this."
Most language is not indexical — "photosynthesis" means the same thing regardless of who says it or when. But indexicals are the connective tissue of communication.
Notes have their own indexicals. "See above" is an indexical — it only works in a specific document layout. Wiki links are closer to proper names, but they can break (dead links) in a way proper names can't.
The design question: should a knowledge system try to eliminate indexicality (canonical IDs for everything) or embrace it (links as contextual gestures)?