Update man page to include FreeBSD-specific details.

While this implements a standards-conforming C11 function, there's
implementation details the programmer needs to know. Include those
here. Make changes inspired by comments on the initial review as well,
though mostly this involves stealing the epoch verbage from
gettimeofday(2). Add myself to authors since I've now changed a
substantial amount of this man page.

lib/libc/gen/timespec_get.3

@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 4, 2016
+.Dd August 10, 2018
 .Dt TIMESPEC_GET 3
 .Os
 .Sh NAME
@@ -39,7 +39,6 @@
 .Lb libc
 .Sh SYNOPSIS
 .In time.h
-.Vt #define TIME_UTC 1
 .Ft int
 .Fn timespec_get "struct timespec *ts" "int base"
 .Sh DESCRIPTION
@@ -50,9 +49,14 @@ function sets the interval pointed to by
 to hold the current calendar time based on the specified time base in
 .Fa base .
 .Pp
-Currently the only supported valid base is
-.Dv TIME_UTC .
-It returns time elapsed since epoch.
+The base
+.Dv TIME_UTC
+returns the time since the epoch.
+This time is expressed in seconds and nanoseconds since midnight (0 hour), January 1, 1970.
+In
+.Fx ,
+this corresponds to
+.Dv CLOCK_REALTIME .
 .Sh RETURN VALUES
 The
 .Nm
@@ -61,16 +65,22 @@ function returns the passed value of
 if successful, otherwise
 .Dv 0
 on failure.
-.\" .Sh ERRORS
 .Sh SEE ALSO
-.Xr clock_gettime 2
+.Xr gettimeofday 2 ,
+.Xr clock_gettime 2 ,
+.Xr time 3
 .Sh STANDARDS
 The
 .Nm
-function conforms to
+function with a
+.Fa base
+of
+.Dv TIME_UTC
+conforms to
 .St -isoC-2011 .
 .Sh HISTORY
 This interface first appeared in
-.Nx 8 .
+.Fx 12 .
 .Sh AUTHORS
 .An Kamil Rytarowski Aq Mt kamil@NetBSD.org
+.An Warner Losh Aq Mt imp@FreeBSD.org