Fix documentation on upgrading when using a Helm chart

What does this MR do?

Document that when updating using a Helm chart helm repo update needs to be run before the helm upgrade ... command. Without that step helm grinds around for a while but doesn't actually update anything.

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
• 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

/cc: @joshlambert