The help text page “Selection Mode” is a bit unclear. Hence I propose the following changes:

Standard selection: 
Current text: "Click in text where you want to position the cursor; click in a cell to make it the active cell. Any other selection is then deselected."
Extending selection: 
Proposal: "This is the default selection mode for text documents. With the keyboard, selections can be performed by Shift+NavigationKey (arrows, Home, End, Page Up, Page Down). With the mouse, click in the text where the selection shall begin, hold the left mouse key, move to the end of the selection and release the mouse key." 

Extending selection:
Current text: "Clicking in the text extends or crops the current selection."
Proposal: "By using the arrow keys or the keys Home and End you can extend or crop the current selection. Clicking into the text selects the text between the current cursor position and the click position. You also can use (with or without Shift key) the arrow keys and the keys Home and End to increase or decrease the current selection."

This following help text could referenced as Related Topic:
“Navigating and Selecting With the Keyboard”