Improve the marker table in markers_api documentation

[trpham]

Prefer to this issue: #7342

Improve the current page: http://matplotlib.org/devdocs/api/markers_api.html. as below:

• Fixed the wrong directional curly quotes by adding backquotes.
• Grouped the marker implies "nothing" together.
• Fit the (numsides, style, angle) entry and its description inside the table.

[anntzer]

I would fold the "(verts, 0)" form together with "verts". Also, did you figure out what all the TICKs and CARETs are about?

[tacaswell]

Can you remove None from the table and add it back as a note under the table? None is the default which is 'nothing', however this table is referred to from other docs for the valid inputs from marker inputs and in those cases None still means 'default', but may not be 'nothing'.

[tacaswell]

This still needs to include the string 'None' (distinct from the object None)

[tacaswell]

Leave this comment at the bottom, no need to draw attention to it.

[anntzer]

Keep them together with the non-filled versions?

It may also make sense to reorder the entries a bit (say square / pentagon / hexagon1/2 / octogon) for example. Also, what is the difference between the two hexagons? Orientation I guess?

[tacaswell]

appveyor failures look unrelated.

[NelleV]

Thanks @trpham for your hard work this semester!

[QuLogic]

Backported to v2.x as 16545df.