chore: sharpen heredoc guidance in MR review instructions

Summary

Rule 19 (Command documentation → Examples) already told authors not to hand-format multi-line strings, but it was thin enough that both a human reviewer and Duo missed the same anti-pattern in !3731: a Long field built from "...\n" + "...\n" + ... concatenation, which produces literal, un-reflowable line breaks in --help output and generated docs.

Compare:

• !3731 (comment 3727240023) — Duo's suggestion caught only the missing example comments (rule 17), not the underlying heredoc violation.
• !3731 (comment 3727312824) — the more useful catch, which reworked the field with heredoc.Docf.

This MR expands rule 19 to:

• Show the hand-concatenated anti-pattern concretely, so it's recognizable even when it "looks fine" in source.
• Explain how to handle a literal backtick inside heredoc content (heredoc.Docf with a %[1]s placeholder), instead of splicing raw and interpreted string literals together.
• Point to internal/commands/mr/note/mr_note_create.go as the canonical reference.

Test plan

• .gitlab/duo/mr-review-instructions.yaml still parses as valid YAML.
• Docs-only change; no Go code touched.