math(3): Use the .Fa macro for function arguments

.Fa is the suitable macro for functions in comparsion to the
.Ar macro, which should be used for commandline arguments.

While here, fix some mandoc warnings.

Reviewed by: imp (earlier version)
Obtained from: OpenBSD (in partial)
Differential Revision: https://reviews.freebsd.org/D31090

(cherry picked from commit c5cbef2f85e6020ef8357b7d3af3ca228a262309)