It is possible to temporarily enable selection modes with ctrl, alt and shift, so this should be mentioned in help. https://help.libreoffice.org/Common/Selection_Mode
Relevant help page: https://help.libreoffice.org/latest/en-US/text/shared/02/20050000.html?DbPAR=SHARED#bm_id3148668 As an update to this issue: 1) The F8 shortcut for "Extending Selection" is no longer valid, since by default LO is assigning F8 to the "Select Cycle" command. In this case "shift" is the modifier. 2) Shift+F8 works for "Adding Selection" and "Ctrl" is the temporary modifier 3) Ctrl+Shift+F8 works for "Block Selection" and "Alt" is the temporary modifier.
Rafael Lima committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/1e7954968f401bf69434c1c53701da335d0a63f2 tdf#107290 Update "Selection Mode" help page