Add docs on additiona ./, .* and / signatures

[rok-cesnovar]

Summary:

The following operator uses are now also supported and need docs:

real / vector
real / row_vector
real / matrix

real .* vector
real .* row_vector
real .* matrix

vector .* real
row_vector .* real
matrix .* real

[rok-cesnovar]

The stanc3 PR was reverted because of lack of Stan Math support for this, so this will have to wait.