[DOCS] Callout to explain that the L2 blocks epoch are NOT the same as the Beacon Chain epochs

[PaulRBerg]

Brief Description of the Docs Request

OP Stack assigns 'epochs' to L2 blocks, but these are not the same as the Beacon Chain epochs.

Description of the Documentation You'd Like

Add an informational callout to explain that the L2 blocks epoch are NOT the same as the Beacon Chain epochs.

Links to Related Pages

https://docs.optimism.io/stack/protocol/rollup/overview#block-production

Additional Context

This duplicity of meaning confused me while reading the docs, and led me to post this question:

ethereum-optimism/developers#416