create "help" text in MS Word

The Screen Tip text is not, in fact, limited to 256 characters. You can include up to about 2080 characters (including line/paragraph breaks). To do this, create your hyperlink, with some dummy display text, then select the field and press Shift-F9. You should see something like:

{HYPERLINK "http://answers.microsoft.com/en-us/office/forum/office_2010-word/create-help-text-in-ms-word/723c4bef-1bbb-4071-8cc3-7b9c37213b7a" \o "Dummy Text"}

Replace 'Dummy Text' with your preferred text, then select the field and press F9 to update it. Your fulsome help-text should now display when you hover over the hyperlink.

Because of the limit on the length of the ToolTip in a hyperlink, you can't use a real hyperlink. The alternative is a macrobutton field made to look like a hyperlink, and that runs a macro to display the popup box.

In a macro you can use the MsgBox function to display text. Although there is some limit on its length, the following demonstration shows that it will handle at least 1000 characters.

Sub ShowText()
    Dim msg As String
    Dim i As Long
    ' 50 characters per line, including the space
    For i = 1 To 20
        msg = msg & "1234567890123456789012345678901234567890123456789 "
    Next i
    MsgBox msg, , "1000 characters in message box"
End Sub

The macrobutton field should have this code:

{ MACROBUTTON ShowText MacroButton that looks like a hyperlink }

Press Ctrl+F9 to insert the field delimiters around the field code, or use the Insert > Quick Parts > Field dialog. To make it look like a hyperlink when the field code is collapsed, select the code (starting from the third word) and apply the Hyperlink character style to it.

By default a macrobutton field requires a double-click to activate the macro, but you can add this code to make it use a single click:

Sub AutoOpen()
    Options.ButtonFieldClicks = 1
End Sub