Improve Javadoc for `getFilename()` and `getFilenameExtension()` in `StringUtils`

[sbrannen]

Currently, the Javadoc for those methods does not explain the outcome for all use cases.

In addition, the generated HTML for getFilename() has a formatting error: it displays "mypath/myfile.txt" → "myfile.txt" instead of "mypath/myfile.txt" → "myfile.txt".