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/sources/commandline/command/build.rst

@@ -19,10 +19,14 @@ Examples
 
     docker build .
 
-This will take the local Dockerfile
+| This will read the Dockerfile from the current directory. It will also send any other files and directories found in the current directory to the docker daemon.
+| The contents of this directory would be used by ADD commands found within the Dockerfile.
+| This will send a lot of data to the docker daemon if the current directory contains a lot of data.
+|
 
 .. code-block:: bash
 
     docker build -
 
-This will read a Dockerfile form Stdin without context
+| This will read a Dockerfile from Stdin without context. Due to the lack of a context, no contents of any local directory will be sent to the docker daemon.
+| ADD doesn't work when running in this mode due to the absence of the context, thus having no source files to copy to the container.