[dirname] fix documentation when using on a full directory path

[Emmanuel Lacour]

Hi,

coreutils 6.10-6 (debian lenny x86 32bit)

when we use "dirname /full/directory/path/" we get "/full/directory/",
but documentation say:

dirname - strip non-directory suffix from file name
                ^^^^^^^^^^^^^^^^^^^^

so either there is a need to update the documentation to say it is
removing the last component (wether it's a dir or a file) or fix dirname
so it returns "/full/directory/path/".