largefiles: fix misleading comments in lfutil instore and storepath
Problem in both cases is cache in largefiles has assigned
meaning - user cache which is additional place to get/put
files. Those two function works on store - the main place
to store largefiles in the repository - .hg/largefiles and
using "cache" to describe it is misleading.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.