Add entry for regexp in migration doc. #11828

Job	Run time
build_js	1m 59s
build_opam (4.14.x)	8m 56s
build_details	2s
build_opam (5.2.x)	5m 13s
build_no_depopts	5m 47s
lezer_parse	52s
build_win32 (x64)	17m 41s
build_posix (debian_trixie, amd64)	15m 41s
run_tests (@citest)	13m 18s
build_posix (debian_bookworm, amd64)	14m 19s
tree_sitter_parse	47s
run_tests (@doctest)	7m 46s
build_posix (ubuntu_oracular, amd64)	15m 49s
update_doc	11m 45s
build_posix (ubuntu_noble, amd64)	15m 27s
run_tests (@mediatest)	12m 19s
build_posix (alpine, amd64)	12m 49s
fetch_s3_artifacts	19s
build_docker_alpine (amd64)	51s
build_docker_minimal (amd64)	1m 59s
build_docker_alpine_minimal (amd64)	57s
build_docker (amd64)	2m 22s
update_release	0s
build_docker_release-minimal	0s
build_docker_release	0s
	2h 46m 58s

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