feat(API): improve DELETE /projects/{project_id}/screenshots/{id} documentation

[Stephen Lumenta (sbl)]

Fixes rubric dimensions: conceptual_clarity, error_documentation, example_quality, frontmatter_metadata, reference_completeness, schema_completeness, self_contained_examples, stable_identifiers, vocabulary_consistency

Part of DEVEX-60

Source-grounded via Phrase-Engineering/strings-app.

Changes

• operationId corrected from screenshot/delete to screenshots/destroy (resource/action convention)
• description expanded from a single vague sentence to a full description covering what the operation does, why you would use it, the string-code id shape, the cascade (markers removed), the feature gate (Attachable Screenshots), and the webhook side effect
• branch parameter replaced inline definition with shared param ref
• 204 response replaced bare ref with inline response including rate-limit headers
• 403 description reworded to accurately reflect Manage permission requirement
• curl example fixed branch from JSON body to query parameter; replaced placeholders with realistic example values; removed incorrect flags for a DELETE with no body
• CLI example replaced angle-bracket placeholders with realistic example values