sphinx: add support for option no cache -E

Sphinx has an option

-E                don't use a saved environment, always read all files

that effectively forces a full rebuild without truly deleting everything. This forces update of label texts, indexes, etc of files that have not changed.

We could add an extra target doc_nocache and/or option DOC_CACHE for this. Personally I prefer the target, since most of the time you want to run normally to save time.