Bug fix and general touch ups for hist3d_demo example (#1702)

[TrishGillett]

• Fixes bug Bug in 3D histogram documentation #1702 by using 'F' when flattening xpos, ypos
• Makes the bin ranges fixed, also as discussed on Bug in 3D histogram documentation #1702
• Adds a docstring (as per MEP12) plus some clarifying comments

[TrishGillett]

I can see that the build has failed, but I've looked at the logs and I'm not sure how the failures are actually related to my changes. I'd appreciate some help understanding this. Thanks in advance!

[jenshnielsen]

The failure on Travis is intermittent and not related to your changes. I have restarted the particular test. I suspect the same is the case on appveyor

[jenshnielsen]

Appveyor is failing due to #5950 since conda have switched to Numpy 1.11

[TrishGillett]

Okay, great, thanks! I see Travis passed this time. :)

[tacaswell]

Thanks!

I believe this is your first mpl contribution, congrats and hopefully we will see you again 😄

[TrishGillett]

Thanks! I've been looking for something approachable to contribute to, and now that I've discovered MEP12 I'm thinking of sticking around here and working on cleaning up some of the other examples. 😊

[tacaswell]

Great! Good documentation is very important and an area where mpl could definitely be doing much better. Any work on the examples is very appreciated.

[tacaswell]

backported to v1.5.x as a8851dd