Improve documentation of View.getCurrentGL()

[martinpernollet]

Retrieving current GL imply synchronisation and locking in JOGL. If the user is not aware of this, he/she might encounter exception if the context is not release as expected.

See Britta's report: https://groups.google.com/forum/?fromgroups=#!topic/jzy3d/P7tkJYml428

Might show projectMouse(...) implementation as an example:

public Coord3d projectMouse(int x, int y) {
    GL2 gl = getCurrentGL();
    Coord3d p = cam.screenToModel(gl, glu, new Coord3d((float) x, (float) y, 0));
    getCurrentContext().release();
    return p;
}