Trac #16570: Animate example looks broken

There is one example used in several places in the documentation of
module `animate`:

{{{
sage: a = animate([sin(x + float(k)) for k in srange(0,2*pi,0.3)],
....:                xmin=0, xmax=2*pi, figsize=[2,1]) # indirect
doctest
}}}

However, at least on my system the individual frames are of slightly
different sizes, ranging from 184×84 to 184×90. And the image looks
pretty strange as well:

[[Image(ticket:16570:broken-example.gif)]]

This seems to be mostly due to the fact that the axis range, described
by `xmin` and `xmax`, doesn't automatically fix a range for `x`.
Instead, it seems as if `x` will automatically range from `-1` to `1`,
but clipped to approximately the axis range.

My suggested remedy is to actually provide a range for `x`, and I'll
upload a branch for this. I'll also address some other documentation and
testsuite issues in this. Some of the tests take longer than one second,
but aren't marked as long. One figure looks so small you can't see
anything. And the information a user receives in response to `animate?`
is pretty sparse, since most examples are at the module level. So I
introduced a cross reference to that.

This ticket is a spin-off from some work I did for #16533.

URL: http://trac.sagemath.org/16570
Reported by: gagern
Ticket author(s): Martin von Gagern
Reviewer(s): Jakob Kroeker, Karl-Dieter Crisman