Add descriptions to i18n messages

[jacobo-dominguez-wgu]

Description

Currently, our i18n message files lack descriptions for a big number of the messages. Adding descriptions is important to provide context for translators, avoid ambiguities, and ensure high-quality translations across all supported languages.

Review all i18n message files (message.js).
For each message, add a clear and concise description explaining its context, usage, and any relevant details (e.g., where it appears, intended audience, tone, etc.).
If a message is self-explanatory, add a brief note to confirm this.
Ensure that no message is left without a description.

Example:
inviteLearners: { id: 'learner.credit.management.empty.state.invite.learners', defaultMessage: 'Invite learners', description: 'Title for the invite learners section', },