Add imatmul documentation

This commit introduces documentation for the imatmul kernels, with extra
focus on the left hand side packing.

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Reviewed-by: Jakub Sujak <jakub.sujak@arm.com>
Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Anton Bondarenko <anton.bondarenko@arm.com>