Identify `wLinkData` components

[Rangi42]

wLinkData is an opaque 1300-byte buffer used for transferring Link Cable data. However, the reason it's 1300 bytes is because it reuses wOverworldMapBlocks. The amount that's actually uses is under 500 bytes. Identifying exactly how it's used would be useful.

Only engine/link/link.asm uses it, buffering different kinds of data into it, so it will need its own UNION to document each unique use.

ClearLinkData does clear all 1300 bytes, so we can't actually make it smaller in pokecrystal itself, but if we document that only the first N bytes are used, then forks can do so safely.

[vulcandth]

I think it is likely already defined in the same union just below it. They likely just need to be collapsed.