It is an Emacs-wide convention to use `RET` when referring to the act of
pressing the return key. Spacemacs seems to follow that convention as well. So
fix some Org files that were deviating from this convention.
In particular, replace usages of `<return>` and `return` with `RET`.
Also update the description of an `M-RET` keybinding[1] based on the docstring
of `ido-select-text`.
[1]: Suggested-by: bmag <bmag@users.noreply.github.com>
