Please document where cache/prebuilds are stored

[KES777]

From documentation save builds for CPAN distributions; I can not figure out where prebuilds are stored.

I want to mount this folder for my Dockerfile, to speedup installation when container is recreated.

[KES777]

For reference, this is ~/.perl-cpm:

RUN  \
  --mount=type=cache,target=/root/.perl-cpm   \
  cpm install  -w 16  --no-test  -L ./local   \
  --show-build-log-on-failure                 \
  --snapshot ./cpanfile.snapshot              \
  --cpanfile ./cpanfile

[bes-internal]

it would be a good idea to display all path in --help