Toolbar button text not changeable: I cannot change the text of the toolbar button "Document Overview On" in transaction ME21N. Is this a special button?
This is a bug in GuiXT, corrected in version 2016 Q3 3.

Workaround:

Either use the tooltip text, e.g.

Text P[Switch On Document Overview] "Doc.Overview"

or the function code e.g.

Text P[/8] "Doc.Overview"