docs: updates documents to describe default target behaviour

[chrsalx]

- What I did

• I updated the references to the --target flag so it now explains what happens when the flag is not passed.

- How I did it
I searched all mentions of the --target flag and replaced the docs where it suited.

- How to verify it
You can test this by running docker build -t demo --progress plain . on any directory with the following image.

FROM ubuntu:latest as stagea
RUN echo "stagea"

FROM ubuntu:latest as stageb
RUN echo "stageb"

You should see something like this in the logs

#6 [stageb 2/2] RUN echo "stageb"
#6 0.097 imageb
#6 DONE 0.2s

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)
Here's a pic of my dad's rottie. She's the loveliest being on earth. ♥️🐶

[chrsalx]

Bumping this.