First, to get help with applescript just open the AppleScript Editor application and look under the help menu. The applescript language guide is in there and other tools. Also under the file menu is "Open Dictionary" which will show you the specific commands for applications.
To see the information about display dialog, open the dictionary of StandardAdditions and use the search field to find the command.
To answer your question, no, you cannot specify the location of a "display dialog" window. There are no parameters in that command for positioning the window, as you'll see in the dictionary. In addition, no other commands will help you either because you can't issue other commands while the dialog window is open because the code pauses while the window is open waiting for a response from the dialog window (like when you press the OK button).
If you need to set the position of a window to display information to a user then you'll need to use some other dialog tool other than "display dialog". You could create your own window in cocoa or google for some alternatives like cocoaDialog, SKProgressBar, and Notification Center.