Clarify which are is modified in mul/div.

[simonbyrne]

My stab at addressing JuliaLang/LinearAlgebra.jl#338.

[mschauer]

While a legal identifier, mulA!2(...) looks to me much like an operation mulA ! 2, not like a function name.

[tkelman]

Aside from picking exactly what the names should be, I think this is a much better direction than what's on master. Mutating a different argument is a different operation, these shouldn't share the same generic function name when their results (side effects) are dramatically different.

[JeffBezanson]

This is orthogonal to the changes on master. The changes on master affect the transposing part, not the mutation part.

[tkelman]

I'm aware. This is an improvement over both master and release-0.6, though it would need to be rebased for the now-different name on master.