TEP-0090: Matrix - Update Pipeline API definition

While adding documentation for `Matrix`, as proposed in [TEP-0090][tep-0090],
found a couple of issues with the existing `Pipelines` documentation. Most
important is that the `Pipeline` API definition is missing some fields, see
[doc][doc]. This commit has the documentation updates that are not directly
related to `Matrix`, but are needed to prepare to add the `Matrix` docs.

In this change, we update the `Pipeline` API definition to reflect all the
existing fields. We also include other fixes in the `Pipelines` documentation,
including removing confusing use of name `parameter` to refer to `field`.

[tep-0090]: https://github.com/tektoncd/community/blob/main/teps/0090-matrix.md
[doc]: https://github.com/tektoncd/pipeline/blob/62c9b0d861bd2ae01263dfa67ba41fb0fa2bc9a9/docs/pipelines.md#configuring-a-pipeline