STYLE: Improve documentation of GetFirstNode()

GetFirstNode third parameter "byHideFromEditor" was not explained
in the doxygen documentation. It did not seem intuitive to have
a parameter that is a pointer when one might expect to specify a
boolean. The new doxygen string explains that this is a 3-state
variable that is either disregarded if the pointer is set to 0,
or the function only returns a node that matches the given value
(hidden from editor or not).

From: Francois Budin <francois.budin@gmail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25151 3bd1e089-480b-0410-8dfb-8563597acbee