Right now the descriptions suggest terrain-rendering is just a subset of procedural-generation. Is it special enough to warrant its own tag?
There are lots of interesting terrain rendering techniques (and thus questions about those techniques) that are possible and which are also wholly independent from the process by which the terrain data was generated. That is, you can totally render a terrain you didn't generate procedurally.
Similarly, procedural generation can cover a whole ton of other areas (the generation of rooms, items, characters, storylines, et cetera). It doesn't just have to be about terrain. Terrain is, in fact, only a pretty small subset of the applications for procedural generation.
Probably what we should do instead is clarify and appropriately-broad the descriptions for both tags, and go over the questions using them to make sure they're still using the tags correctly.