Rollup merge of #105245 - RalfJung:align_to, r=Amanieu

attempt to clarify align_to docs

This is not intended the change the docs at all, but `@workingjubilee` said the current docs are incomprehensible to some people so this is an attempt to fix that. No idea if it helps, so -- feedback welcome.

(Please let's not use this to discuss *changing* the spec. Whoever wants to change the spec should please make a separate PR for that.)