build/windows: don't use wrong version number

We were using a global temp file while setting the NODE_VERSION
environment variable. This resulted in simultaneous builds swapping
version numbers on occasion.

This patch removes the use of a temp file for this.
doc: Fix readable.unshift() example

Slice the portion of the buffer to unshift back into the read queue
doc: Update to reflect new _read() interface
doc: Clarify advisory-ness of stream._read() argument
doc change for Readable._read()