Dynamically generates a dialog from a help node.
gui.showHelpDialog([<path> [, toggle]])
path ... path to help node
toggle ... decides if an already open dialog should be closed
(useful when calling the dialog from a key binding; default: 0)
help node
=========
each of <title>, <key>, <line>, <text> is optional; uses
"/sim/description" or "/sim/aircraft" if <title> is omitted;
only the first <text> is displayed
<help>
<title>dialog title<title>
<key>
<name>g/G</name>
<desc>gear up/down</desc>
</key>
<line>one line</line>
<line>another line</line>
<text>text in
scrollable widget
</text>
</help>