doc: Re-organize and improve Java Interface documentation. * doc/interpreter/java.txi: Add usejava, isjava functions, debug_java, java_matrix_autoconversion, java_unsigned_autoconversion to docs. Re-organize to put functions in logical order. Add new "Dialog Box Functions" node. * doc/interpreter/system.txi: Remove usejava from list of system functions. * libinterp/octave-value/ov-java.cc(FjavaObject, FjavaMethod, Fjava_unsigned_autoconversion): Improve docstrings. * scripts/java/javaArray.m: Tweak docstring. * scripts/java/listdlg.m: Wrap long lines in docstring. Improve code example in docstring.