docs: clarify preservePath behavior

[vibhor-aggr]

Clarifies what preservePath: true does for uploaded file metadata.

The README currently says it keeps the full path instead of the base name, but it does not explain where that path appears or that it is client-supplied input. This adds wording that:

• identifies file.originalname as the field affected by preservePath;
• notes that Multer does not create directories or sanitize the path;
• warns applications to normalize or validate file.originalname before using it in a custom filename or storage engine.

Refs #846.

Verification:

• npm test (72 passing)
• npm run lint with HOME pointed at the task workspace because the sandbox home is read-only
• git diff --check HEAD~1..HEAD