Rename Dirty.docs to Dirty._docs
Rename Dirty.docs to Dirty._docs

Users should not access the document array directly.

felixge/node-dirty,simudream/node-dirty,felixge/node-dirty,beni55/node-dirty,simudream/node-dirty,hagb4rd/node-dirty,hagb4rd/node-dirty,isaacs/node-dirty,beni55/node-dirty