axes.arrow and FancyArrow documentation and concerns

Hi all,

I just sent in a pull request #941 to update the documentation for axes.arrow, because width,head_width,head_length, etc weren't documented, but I ran into a couple of concerns that I thought to discuss:

1. the default width of 0.001 seems absurdly small. 0.05 seems better. You can tell there's actually an arrowhead there with reasonable x- and y-limits.

2. I believe that length_includes_head ought to default to True. especially since the doc says: Draws arrow on specified axis from (*x*, *y*) to (*x* + *dx*, *y* + *dy*), one would expect by default the tip of the arrow to end at *x* + *dx*, *y* + *dy*

3. for the 'shape' arg, 'left' and 'right' seem to be backwards to me. Given that the arrow defines a vector direction, say \hat{x}, then 'left' to me means only the part of the arrow in the +y half of the plane should be drawn.