Actually, the manual has the full story: Copy text from a mail message 1. Tap the message at the point from where you want to start copying. 2. Press and hold the shift key, and use the arrow keys to select text. 3. Press ctrl + C to copy the text, and ctrl + V to paste it to another location.
The current method could remain as a shortcut, but a toolbar button for toggling, as in all the PDF reades that everyone already knows, would avoid the need for actually requiring people to read the manual. There seems to be space available in the toolbar.