docs: warn about build data tx & ADD w/o context

This updates the documentation to mention that:
1. a lot of data may get sent to the docker daemon if there is a lot of
data in the directory passed to docker build
2. ADD doesn't work in the absence of the context
3. running without a context doesn't send file data to the docker
daemon
4. explain that the data sent to the docker daemon will be used by ADD
commands
docs: build: ADD copies just needed data w/ full path
Merge pull request #845 from unclejack/841-update_docs_no_add_without_context

841 - docs: warn about the transmission of data to the docker daemon & ADD without context