Clarify that a `rawdata` directory is not a prescription

[sappelhoff]

https://github.com/bids-standard/bids-specification/blob/40f4fbb0649a8dab65f4327df342251ad841bfb8/src/common-principles.md?plain=1#L289-L291

although this sentence might make it seem as if it were. (re: discussion at sprint)