Improved MA documentation

+ Added a bit of background, linking to Wikipedia
+ Added more comments to the code examples
+ Example chart code is no longer global - @tunerscafe would be good to do the same for the other examples.
+ SVG tap indicates that this an output rather than an input

I know this hasn't changed in this commit, but should the `<xmp>` tags be replaced with `<pre>` or `<code>` tags? @tunerscafe 

`<xmp>` tags are obsolete: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/xmp