Documentation: always respect core.worktree if set

Documentation: always respect core.worktree if set

The value of core.worktree in a ".git/config" is honored even when it
differs from the directory that has the ".git" directory as its
subdirectory.  This is likely to be a misconfiguration, so warn users
about it.  Also, drop the part of the documentation that incorrectly
claimed that we ignore such a misconfigured value.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>