docs: clearer distinction between `.java' with javac and with gcj

docs: clearer distinction between `.java' with javac and with gcj

* doc/automake.texi (Java support): Rename this node ...
(Java support with gcj): ... to this, and change its title from
"Compiling Java sources" to "Compiling Java sources using gcj".
(@detailmenu, @menu, @cindex): Update and make more precise.
(Java): Change the title of this node from simply "Java" to
"Java bytecode compilation".
(@detailmenu, @menu, @cindex): Update and make more precise.