Document complex math library.

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+.\"	$OpenBSD: clog.3,v 1.1 2011/07/20 17:50:43 martynas Exp $
+.\"
+.\" Copyright (c) 2011 Martynas Venckus <martynas@openbsd.org>
+.\"
+.\" Permission to use, copy, modify, and distribute this software for any
+.\" purpose with or without fee is hereby granted, provided that the above
+.\" copyright notice and this permission notice appear in all copies.
+.\"
+.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+.\"
+.Dd $Mdocdate: July 20 2011 $
+.Dt CLOG 3
+.Os
+.Sh NAME
+.Nm clog ,
+.Nm clogf ,
+.Nm clogl
+.Nd complex natural logarithm
+.Sh SYNOPSIS
+.Fd #include <complex.h>
+.Ft double complex
+.Fn clog "double complex z"
+.Ft float complex
+.Fn clogf "float complex z"
+.Ft long double complex
+.Fn clogl "long double complex z"
+.Sh DESCRIPTION
+The
+.Fn clog ,
+.Fn clogf
+and
+.Fn clogl
+functions compute the complex logarithm to the base
+.Ms e
+(2.718...) of
+.Fa z .
+.Pp
+If
+.Fa z
+= x + iy, then
+.Bd -literal -offset indent
+clog(z) = log(sqrt(x^2 + y^2)) + i atan(y / x).
+.Ed
+.Sh RETURN VALUES
+The
+.Fn clog ,
+.Fn clogf
+and
+.Fn clogl
+functions return the complex logarithm to the base
+.Ms e
+of
+.Fa z
+with imaginary part in the interval
+.Bq -Pi, Pi ,
+and unbounded real part.
+.Sh SEE ALSO
+.Xr cexp 3 ,
+.Xr cpow 3
+.Sh STANDARDS
+The
+.Fn clog ,
+.Fn clogf
+and
+.Fn clogl
+functions conform to
+.St -isoC-99 .