docs: clarify -r replacement flag - #3500

Kept typing rg -rn PATTERN out of grep habit, expecting recursive search with line numbers — but ripgrep reads -r as --replace and swaps every match for the literal n. Recursion is already the default. This change just documents that gotcha in the guide and the --replace help, with contrasting examples. No behavior change. Validated with formatting checks, 118 binary tests, 10 replacement integration tests, and by generating and inspecting the help/man output.