Doc: improve doc on markup command writing (issue157133)

[nicolas . sceaux]

Reviewers: ,

Message:
Hi,

This patch improves (as I hope) the documentation on markup command
definition, by using more complete examples.  It also tries to address
an issue that was raised on -devel, concerning the different syntax
between built-in and user-defined markup command definition.

As I'm not (obviously) a native English speaker, comments on style are
most welcome.

Nicolas


Description:
Doc: improve doc on markup command writing

Please review this at http://codereview.appspot.com/157133

Affected files:
  M Documentation/extending/programming-interface.itely
  scm/markup.scm