Bug 377659 - Remove multiple editor tabs preference

Remove documentation for the removed preference setting.
Also fix the link in the context help hat lead to the documentation
of the *Text* Editor preference page instead of the Editor preference
page.

Change-Id: Iac2b457c810822c28fda3f881ff16389ad04a2eb
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.common/+/187925
Tested-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-by: Lars Vogel <Lars.Vogel@vogella.com>