fix: clarify target-dir mismatch for rules-dir

[immanuwell]

fixes a misleading config-reloader error when --target-dir is used with legacy --rules-dir.

right now the binary says --watched-dir even if the user passed only --rules-dir. kinda confusing, since --rules-dir is still supported for compat.

repro:
go run ./cmd/config-reloader -rules-dir=/tmp/a -target-dir=/tmp/x,/tmp/y

before:
--target-dir count (2) must match --watched-dir count (1)

after:
--target-dir count (2) must match watched directory count (1) from --watched-dir/--rules-dir

also adds a tiny regression test and updates the generated flag doc.

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}