Update doc for "Changing case in a filename"

The issue #830 has supported "git mv foo.txt Foo.txt --force",
so the document needs to be updated too.

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Update doc for "Dealing with filename case conflicts"

There is one possible way to deal with it, so update it.

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>