Clarify that a rawdata directory is not a prescription

[sappelhoff]

bids-specification/src/common-principles.md

Lines 289 to 291 in 40f4fbb

	In this example, where `sourcedata` and `derivatives` are not nested inside
	`rawdata`, **only the `rawdata` subdirectory** needs to be a BIDS-compliant
	dataset.

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

[effigies]

I suppose we could just add

(Note that rawdata here is an arbitrarily-chosen name. BIDS specifies the contents of a dataset, not the names of directories containing datasets.)