--update-snapshots command line option flag is misleading, as it actually updates releases too

Had a discussion with Brett Porter, Brian Fox, and Benjamin Bentmann on IRC about how the --update-snapshots command line option is misleading. I, as well as some of my students, we using -up, expecting downloads that previously failed to be tried again. However, that flag only checks for metadata updates to see if a newer version of the plugin has been released. What we were after is the -U flag (synonymous with --update-snapshots) which also checks again remotely for SNAPSHOT and RELEASE downloads that previously failed.

Brett and Brian suggested that we either deprecate the old --update-snapshots long option, but that's not entirely possible since the short and long options are tied together. So, as a compromise solution, I'm indicating that the --update-snapshots option flag (name) may go away in the future and adding a migrate-to option called --update-all which was suggested in the above IRC conversation. Functionally, this long flag performs the same operation as --update-snapshots, but the name much more effectively communicates that it updates all artifacts from remote repos, not just snapshots.