Rename -Zparse-only.

I was surprised to find that running with `-Zparse-only` only parses the
crate root file. Other files aren't parsed because that happens later
during expansion.

This commit renames the option and updates the help message to make this
clearer.

compiler/rustc_driver_impl/src/lib.rs

@@ -425,7 +425,9 @@ fn run_compiler(
                 return early_exit();
             }
 
-            if sess.opts.unstable_opts.parse_only || sess.opts.unstable_opts.show_span.is_some() {
+            if sess.opts.unstable_opts.parse_crate_root_only
+                || sess.opts.unstable_opts.show_span.is_some()
+            {
                 return early_exit();
             }