Docs: Add note to also index the database if doing async index

What does this MR do?

We've had multiple customers do the async index with Elasticsearch, but they only complete the repository index and don't index the database. This note will hopefully clarify these steps so other customers won't have the same issue.

What are the relevant issue numbers?

For internal use, here are two recent tickets where this came up: https://gitlab.zendesk.com/agent/tickets/108849 https://gitlab.zendesk.com/agent/tickets/110295

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards