Add `diagind()` Method on Matrix Dimensions Only

[RoyiAvital]

Currently, diagnind() requires the matrix instance in order to calculate the indices.
It would be great to have a version which requires only the dimensions of the matrix (m, n) to calculate the indices.

[Luis-Varona]

I feel like this is actually a lack of documentation, @RoyiAvital—we certainly already have a diagind(m::Integer, n::Integer, k::Integer=0) method implemented here, and it is indeed publicly exposed:

julia> using LinearAlgebra

julia> diagind(5, 7)
1:6:25

The documentation you pointed ou leaves something to be desired, though; it could (and probably should) be updated to reflect this.