Docs - Update truncate design guidelines

[edonehoo]

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Follow-up to https://github.com/patternfly/patternfly-org/pull/4703

• Add elements image OR remove the section
• Change “front-line”, “mid-line”, and “end-line” to align with React example naming: “default/end”, “start”, “middle”
• Some of the content considerations is more appropriate for the usage section