Clarify detect_machine event and variable values

[Lucas-FManager]

Summary

• Clarifies that detect.params.event and ${detect_result} are separate surfaces with different value sets and casing.
• Splits callback event values into detection outcomes and lifecycle markers.
• Adds the missing ${detect_result} values detecting and error, and fixes a small SWML typo in the same section.

Root cause

The page listed callback events and showed lowercase ${detect_result} examples without explicitly saying these are separate contracts. That made READY, NOT_READY, and finished look like possible ${detect_result} values even though they are callback lifecycle events only.

Code shape

One MDX documentation page changed. The update is intentionally local to the detect_machine reference page; no generated code, schemas, runtime behavior, or wire-format casing changed.

Security assessment

Low risk: documentation-only change. No executable code, dependency, auth behavior, network request handling, secrets, or customer data path changed.

Verification

• rg -n "detect_result|READY|NOT_READY|finished|SWML|SMWL|machine \\| human \\| fax \\| unknown \\| detecting \\| error" fern\\products\\swml\\pages\\reference\\methods\\calling\\detect_machine.mdx
• git diff --check

Note: I did not run the full Fern docs check locally because this workspace is a sparse checkout and does not have the full docs dependency/config tree installed.

Fixes #333