Move _all.do -> all.do and slightly update docs.

all.do's main job was to print a "nothing much to do" message after
running.  Nowadays it actually does do stuff, so we can remove the
warning, making _all.do redundant.

all.do

@@ -1,2 +1,9 @@
-redo-ifchange _all
-echo "Nothing much to do.  Try 'bin/redo -j10 test'" >&2
+if [ "$1,$2" != "all,all" ]; then
+	echo "ERROR: old-style redo args detected: don't use --old-args." >&2
+	exit 1
+fi
+
+# Do this first, to ensure we're using a good shell
+redo-ifchange redo/sh
+
+redo-ifchange bin/all docs/all