docextract_to_xml.py: Add '&rArr;' to the list of escape sequences.
docextract_to_xml.py: Add '&rArr;' to the list of escape sequences.

	* tools/defs_gen/docextract_to_xml.py: It represents a right arrow and
	some C API documentation uses it.

GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm