Document the Graphviz macro so that ![[MacroList]] macro provides useful information

[Emmanuel Blot]

A Wiki page such as WikiMacros gives this kind of meaningless output...

[[graphviz.dot/png]]

    Blah, blah, blah.
[[graphviz.dot/jpg]]

    Blah, blah, blah.
[[graphviz.dot/gif]]

    Blah, blah, blah.
[[graphviz.dot/svg]]

    Blah, blah, blah.
[[graphviz.dot/svgz]]

    Blah, blah, blah.
[[graphviz.dot]]

    Blah, blah, blah.
[[graphviz.neato/png]]

    Blah, blah, blah.
[[graphviz.neato/jpg]]

    Blah, blah, blah.
[[graphviz.neato/gif]]

    Blah, blah, blah.
[[graphviz.neato/svg]]

    Blah, blah, blah.
[[graphviz.neato/svgz]]

    Blah, blah, blah.
[[graphviz.neato]]

    Blah, blah, blah.
[[graphviz.twopi/png]]

    Blah, blah, blah.
[[graphviz.twopi/jpg]]

    Blah, blah, blah.
[[graphviz.twopi/gif]]

    Blah, blah, blah.
[[graphviz.twopi/svg]]

    Blah, blah, blah.
[[graphviz.twopi/svgz]]

    Blah, blah, blah.
[[graphviz.twopi]]

    Blah, blah, blah.
[[graphviz.circo/png]]

    Blah, blah, blah.
[[graphviz.circo/jpg]]

    Blah, blah, blah.
[[graphviz.circo/gif]]

    Blah, blah, blah.
[[graphviz.circo/svg]]

    Blah, blah, blah.
[[graphviz.circo/svgz]]

    Blah, blah, blah.
[[graphviz.circo]]

    Blah, blah, blah.
[[graphviz.fdp/png]]

    Blah, blah, blah.
[[graphviz.fdp/jpg]]

    Blah, blah, blah.
[[graphviz.fdp/gif]]

    Blah, blah, blah.
[[graphviz.fdp/svg]]

    Blah, blah, blah.
[[graphviz.fdp/svgz]]

    Blah, blah, blah.
[[graphviz.fdp]]

    Blah, blah, blah.
[[graphviz/png]]

    Blah, blah, blah.
[[graphviz/jpg]]

    Blah, blah, blah.
[[graphviz/gif]]

    Blah, blah, blah.
[[graphviz/svg]]

    Blah, blah, blah.
[[graphviz/svgz]]

    Blah, blah, blah.
[[graphviz]]

    Blah, blah, blah.

[Emmanuel Blot]

I think it would make more sense to define a unique "graphviz" macro and specify the processor and output format as argument, rather than generating virtual macro names.

[Peter Kropf]

(In [1166]) Changed get_macro_description to only return a description for graphviz. All the other permutation will not have a description. Fixes #284.