Cannot clear "all" caches

[thejohnfreeman]

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Ubuntu 18.04
• Poetry version: 1.0.0a3

Issue

poetry cache clear --all still expects a cache name (and there seems to be no commands for listing cache names).

$ poetry cache clear --all

[CannotParseArgsException]
Not enough arguments (missing: "cache").

[thejohnfreeman]

Judging from this comment, there is a cache named pypi that has to be given. Perhaps the help message could be improved instead?

[Alexander171176]

Is it possible to explain to people using a code example ??
for example:
poetry cache clear PyPi --all
Delete 86 entries? (yes/no) [yes] yes
yes.