Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

mdoc: fix column names, indentation, column separation within each row, and
quotation. Also make sure we have the same amount of columns in each row as
the number of columns we specify in the head arguments.

Reviewed by: brueffer

Implement csqrtl().

Implement and document csqrt(3) and csqrtf(3).