Fix review comments on ember/template-no-deprecated

[Copilot]

Addresses open review comments from the most recent commit on the ember/template-no-deprecated rule.

Docs (docs/rules/template-no-deprecated.md)

• @arg deprecation checking is already implemented via GlimmerAttrNode — remove the "Not covered" note and add it to the covered syntax table

Tests (tests/lib/rules/template-no-deprecated.js)

• Replace multi-line template literals with single-line strings + explicit \n to eliminate invisible whitespace (makes column positions easier to debug)
• Drop redundant — no error suffixes from valid comments
• Add sub-expression test case: {{fn (deprecatedHelper)}}

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