update --data docs

In particular, be explicit about what transformations we perform on the argument. We mention \n and tokens, but we don't mention adding the dot, From_ stripping, line endings, etc. When adding it, explicitly mention options for turning off that behavior (--no-data-fixup, --no-strip-from, etc).
- --no-data-fixup - may need to change this, but no matter what change "massage" to manipulate, or whatever word we end up using to define the behavior in --data section