Clarify what the ARG instruction actually does during build

[homm]

Previously, the section described:

• how to declare an ARG,
• how to provide default values,
• how users can override it using --build-arg,
• and how its scope is limited to the build stage.

However, the documentation never explicitly stated how ARG behaves inside the build process.

This change improves the ARG documentation by explaining what the instruction actually does during a Docker build.