improvements for reference materials

[lmccart]

• createCanvas says "Appends to the container node if one is specified, otherwise appends to body..." could use a little more clarification and links to relevant sections in overview tutorial. and example of this in the ref.
• p5.dom page: include some info about appended items following flow of the page, how to use position() to absolutely position them. more generally, need to finish up the overdue "p5.dom: beyond the canvas" tutorial
• p5.Element.style(): a little more info, should it be a string or something else (ex: fingers.style('opacity', 0.5); vs mylink.style('color', '#00ff00');)
• reference may need to include p5.dom and p5.sound API, it's hard to find them otherwise. or else clear link to these pages.
• createVideo, createSound: explanation for using an array for input src as option instead of single string src
• p5.Color
• add info/documentation for static functions in p5.Vector

[lmccart]

adding documentation for p5.Vector per https://github.com/lmccart/p5.js/issues/355

[marti1125]

Hi @lmccart could you tell me more about the first task, please

[lmccart]

hi @marti1125. this one has already been addressed actually, thanks!