Specs don't explain that annotations require a padding when the property group size approaches 255

Reading the JS neuroglancer code for parsing the properties of annotations, I noticed that due some WebGL limitations, a modulo-4 byte padding is expected when the property group size approaches 255 bytes: https://github.com/google/neuroglancer/blob/249f866954c4069292c121eaa0592326b7277814/src/annotation/index.ts#L384

This is not mentioned [in the specs](https://github.com/google/neuroglancer/blob/master/src/datasource/precomputed/annotations.md), but I think it should, since otherwise an implementation for writing to the precomputed format may not cover this edge case.