docs: explain that `jj root` is synonymous with `jj workspace root` #388

Job	Run time
nix flake (ubuntu-24.04)	7m 18s
check (clippy)	2m 45s
check (cargo-deny, advisories)	24s
nix flake (ubuntu-24.04-arm)	7m 6s
check (codespell)	7s
nix flake (macos-14)	10m 11s
check (doctests)	2m 31s
check (cargo-deny, bans)	25s
check (latest mkdocs, optional)	11s
check (protos)	27s
check (mkdocs)	13s
check (cargo-deny, licenses)	26s
check (cargo-deny, sources)	24s
check (rustfmt)	18s
check (zizmor)	31s
build (no git)	1m 12s
test (linux-x86_64-gnu)	7m 14s
test (linux-aarch64-gnu)	7m 38s
test (macos-x86_64)	12m 22s
test (macos-aarch64)	4m 28s
test (windows-x86_64)	16m 36s
	1h 22m 47s

Provide feedback