IISPH docs are confusing #911
Description
In the IISPH docs (#751), this part is a bit confusing.
`d_{ij}` is actually only defined for `i != j`. If we set `j = i`, we get zero, because `\nabla W_ii` is zero. We do not get the `d_{ii}` that is defined as the first sum.
I think it would be less confusing if the second sum was written as `\sum_{j \neq i}` with a comment below "note that the second sum does not contain `i`, as `\nabla W_{ii} = 0`".