Update description for label docs (created if doesn't exists)

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What does this MR do and why?

Closes #454576

Update labels and add_labels description to include info that a label will be created if it doesn't exist

Changes made in:

• issues.md

  • Update issue
    • line 1200
    • line 1209

• merge_requests.md

  • Create MR
    • line 1310
  • Update MR
    • line 1455
    • line 1462

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