Add complex number support to vecdot

[rgommers]

Closes gh-356 (where the definition x1^H x2 was decided).

[kgryte]

I'll review and update this PR shortly.

[kgryte]

Updated to reflect extended description conventions used for other linalg APIs. The description is functionality equivalent to expressing the vector product as a matrix product of two column vectors involving a conjugate transpose and is thus aligned with gh-356.

As consensus was previously established to define the complex dot product in terms of the conjugate transpose, will merge. Thanks, @rgommers!