Improve build id documentation

[patrobinson]

We had an escalation from a customer asking:

we're trying to understand what happened with one of our jobs where it seems to have skipped a build number.

We made a change to our system some time ago which means we can no longer guarantee a gapless sequence of build IDs.

This change updates the documentation to replace the language such as "sequence" and "This number increases by 1 with every build" with "monotonically increasing" to more accurately reflect the current state.

I had to take a side quest to get the GraphQL schema docs updating.

[buildkite-docs-bot]

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