Amend description to reflect empty string result for windows drive

[Philzen]

Not sure if this is is a bug or a feature – what i can confirm is that in v2.4 it worked as it said in the description:

@Test void returnsHomeWithTrailingSlash_forHomeWithTrailingSlash() {
    assertThat(FilenameUtils.getFullPathNoEndSeparator("C:")).isEqualTo("C:");
}

After upgrading to 2.15.1, i realized my test suite failed, the behaviour is now:

@Test void returnsHomeWithTrailingSlash_forHomeWithTrailingSlash() {
    assertThat(FilenameUtils.getFullPathNoEndSeparator("C:")).isEqualTo("");
}

This PR proposes to reflect this change in the javadoc.

[elharo]

You've pointed to existing bad API design. getFullPathNoEndSeparator("C:") should return "" if the argument is a Unix path and "C:" if that path is a Windows path. Of course, it's just a string, and we have no way of knowing whether that's a Windows path or a Posix path. In this case, best guess is Windows since "C:" is very common in Windows and very uncommon on Unix, but it's still a guess. Possibly we should deprecate this method and rethink the API here.

[Philzen]

Possibly we should deprecate this method and rethink the API here.

@elharo Agreed. For the time being, any objection merging this PR to bring the JavaDoc inline with the method's actual behavior?