Improve 'cache' command description

[ako80218]

Improve command short and long description in
https://github.com/wp-cli/cache-command/src/Cache_Command.php

wp cache | Manage the object cache.

Proposed short description:
Manipulates the WP Object Cache.

Proposed long description:
Use a persistent object cache drop-in to persist cache values between requests.
WP Object Cache

cc:@hearvox