Run mkdocs only after generating the rest of the doc files.

This avoids a mkdocs bug where mkdocs will (rarely) see redo temp files
in its directory list, but they're gone by the time it tries to open
them, so it aborts with an exception.

Documentation/all.do

@@ -1,3 +1,9 @@
 redo-ifchange doc.list
 sed 's/\.md/.1/' <doc.list |
-xargs redo-ifchange mkdocs
+xargs redo-ifchange
+
+# mkdocs foolishly tries to process every file in this directory, which
+# leads it to try to open temp files produced by the above redo-ifchange
+# if it runs in parallel with those jobs.  So don't run it until they
+# finish.
+redo-ifchange mkdocs