Doc: best practices for language definition files

The list is added in the reference, not the tutorial. The tutorial is
read once, after that the people use the reference.

https://bugzilla.gnome.org/show_bug.cgi?id=652782
Complete lang-reference documentation

- Like <match>, <start> and <end>, the <define-regex> tag can have regex
options attributes (e.g.  "extended" or "case-sensitive").

- Context classes (string, comment, no-spell-check)

- The "original" attribute for a reference context.

https://bugzilla.gnome.org/show_bug.cgi?id=652782