Various improvements to jit documentation Touches one of the tutorial examples, which is also used as part of the testsuite: successfully bootstrapped & regrtested on x86_64-unknown-linux-gnu (Fedora 20).
OK for trunk? (am not yet officially blessed as the JIT maintainer, so I require review for the non-obvious patches) I'd regenerate the autogenerated libgccjit.texinfo when committing.
