From PMS - Installation commands - dodoc: For EAPIs listed in table 12.8 as supporting -r, if the first argument is -r, any subsequent arguments that are directories are installed recursively to the appropriate location; in any other case, it is an error for a directory to be specified. Portage doesn't error out if encountering a directory and -r isn't specified with dodoc.
You can set PORTAGE_ELOG_CLASSES="${PORTAGE_ELOG_CLASSES} qa" in make.conf in order to have the relevant QA Notice logged. *** This bug has been marked as a duplicate of bug 356389 ***