Fix minor doc errors

[amatulic]

In vnf.scad:

• Corrected indentation in vnf_vertex_array() documentation, which was causing all information about triangulation styles and textures to be suppressed.
• Changes to resolve vnf_vertex_array() strange behavior #1767
  • Added short note to documentation about the effects of $fa and $fs when generating rows of points, possibly resulting in a non-rectangular array, in which case vnf_tri_array() can be used instead.
  • Modified non-rectangular assert error to mention vnf_tri_array() as an alternative.

In paths.scad:

• Corrected rendering of simplify_path() example so that the objects aren't cropped by the image boundary.

In attachments.scad:

• Spelling, grammar cleanup only

In shapes3d.scad:

• Inserted newlines in front of every assert message
• Minor wordsmithing / spelling / grammar