Possible documentation improvement

[mmikhasenko]

Currently the doc string of @animate says

p = plot(1)
anim = @animate for x=0:0.1:5
    push!(p, 1, sin(x))
end
gif(anim)

but pushing (push!) to a plot, I think, is not the most common approach,
and it always takes me a few minutes to figure out how to put several independent plots into a gif sequence.

I propose replacing the example by

anim = @animate for x_max in range(0,2)
    plot(0, x_max, sin)
end
gif(anim, fps=5, loop=1)

Also adding better docstring to gif should be part of the same PR,
currently

• parameter loop is not clear, why int as well as
• variable_palette=false
• verbose=false
• show_msg=true

I'll take care of it once have a moment

[BeastyBlacksmith]

The reason, why one should prefer push! here, is that it can be quite a bit faster, since it doesn't need to create a whole new plot each iteration.