Fix documentation about TaskRunSpecs within PipelineRun

[jlpettersson]

Changes

• Name of pipelineTaskName is fixed so it corresponds with the implementation
• Other minor incorrectnesses are fixed as well
• Move the section to be in the same location in the document as in the menu
• Fix broken links

/kind documentation

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Release Notes

- Fix documentation about TaskRunSpecs within PipelineRun

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/lgtm