Revhistory: new docbook tag managed.

[Sylvestre Ledru]

Hello,

To improve the Scilab documentation, we introduced, for the release
5.4.0, the management of the tag revhistory [1] which allows to keep
track of changes in the documentation.

This must be used to document changes of functions in cases like:
* In which version of Scilab the function has been introduced
* In which version did the behavior of the function changed
* In which version the profile changed
* When it has been tagged as obsolete
...

Please consider using it.

For example, how it looks with the function comet3d:
http://codereview.scilab.org/#patch,sidebyside,4122,4,scilab/modules/graphics/help/en_US/3d_plot/comet3d.xml

Thanks,
Sylvestre

[1] http://www.docbook.org/tdg5/en/html/revhistory.html
Note that the tags date and author are not managed on purpose.