docs: explain oneChangePerToken whitespace in diffWords - #706

Refs #698.

Clarify in the README and oneChangePerToken option documentation that diffWords's default whitespace-insensitive mode retains overlapping whitespace between tokens. Include the reported example and point callers who need separate whitespace tokens to diffWordsWithSpace.

This documents existing behavior; it leaves the runtime semantics unchanged.

Validation on Node 24.19.0: build, lint, type checks and ESM/CommonJS examples pass; 45 word-diff tests pass. With the project's Babel preload and an LF README, the full suite has 315 passes and the same three failures on base and patch: two Windows file-mode assertions and a hunk-test timeout. The initial yarn test run stopped loading the README example from a CRLF checkout.