getelementptr inbounds clarifications (#65478)

- For a long time I assumed that `inbounds` means "in-bounds of a *live*
allocation". @nikic told me that is not correct. I think this definitely
needs clarification in the docs.
- The point about successively adding the offsets to the current address
confused be because it talked about the successive addition of "an"
offset -- which one? My interpretation was, the total accumulated offset
computed in the previous step. But @nikic told me that's not correct,
adding each offset individually has to stay in-bounds for each step. I
hope by saying "each offset" this becomes more clear; I then also change
the previous bullet to use the same terminology.