Improve command description on 'transients'

[mitraval192]

Improve command description text:

https://github.com/wp-cli/cache-command/blob/master/src/Transient_Command.php

Current description: Manage transients.

Proposed description:
Manage WordPress transient and standardized way of storing cached data in the database temporarily base.

Example: See the WordPress Transients_API reference.