Documentation bugs in multiset

Stream: general

[Robert Maxton]

Before I forget -- it seems some of the source links in multiset go to the wrong places. Forexample, docs#multiset.sum_add links to https://github.com/leanprover-community/mathlib/blob/de648fd51be82c29292e25a5912fd492683ef876/src/algebra/big_operators/multiset.lean#L70, which is prod_add; docs#multiset.prod_add

[Robert Maxton]

... in fact the problem appears to be mostly with sum_<foo> in general

[Reid Barton]

https://leanprover-community.github.io/mathlib_docs/attributes.html#to_additive

[Reid Barton]

There's no separate source for sum_add, it is generated automatically from prod_add.