net: add comments to phy APIs

Added a comment on the limitations of phy_find_by_mask API when scanning
MDIO buses with multiple PHYs present.  Added short descriptions to the
other APIs in phy.h for consistency.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>