Add documentation for the element states during a build

Background

I can not find any obvious documentation that states what all of the different terminology means when the elements of a build are first being evaluated. There are multiple different terms listed such as:

• "waiting"
• "downloadable"
• "fetch needed"

Task description

I feel these should be documented as it is not obvious to new users what state something is in.

• Add documentation that describes what each state means

Acceptance Criteria

When the buildstream documentation has sufficient information regarding the states of elements

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