nodemap: move the option for mmap usage to storage.revlog.nodemap.mmap The option is stay experimental as long as the main feature is. Differential Revision: https://phab.mercurial-scm.org/D8421

nodemap: move and update the commend about persistence being experimental The comment was at the wrong place (on the developer option instead of the activation switch). So we move it at the right location and update it. Differential Revision: https://phab.mercurial-scm.org/D8420

nodemap: move the main switch to the `format` section The config to enable persistent nodemap is now `format.use-persistent-nodemap`. However the option remain marked as experimental because it only improve performance for people using the rust extensions. Differential Revision: https://phab.mercurial-scm.org/D8419