Encourage user to revert the yes-node patch

The documentation gives the procedure to get a yes node in order to test a protocol proposal/migration/… in: http://tezos.gitlab.io/developer/proposal_testing.html#patch-the-shell-to-obtain-a-yes-node . However, it does not describe how to revert the patch to get a "normal" node afterward. This may lead to unexpected errors (such as #2215 (closed)).

The documentation should state that, after the experiments are done, one should restore the node state using patch -p1 --reverse < scripts/yes-node.patch. (Note that one can also use git to revert the changes, using git checkout . (after making sure that no other changes were done) or, more specifically, using git checkout src/lib_crypto/signature.ml.)