Don't try to recover namespace with unloading disabled

Having 'disable-unload!' or 'disable-reload!' in a namespace could
cause errors when trying to "recover" the namespace mappings after a
failed reload.
Warn about breaking alaises in disable-unload/reload docstring
Add inline headings to 'Warnings' in README
Fix link to Ring in README
Describe no-recover-attempt for reload-disabled in README