[2021.2] Updates to documentation before release - GSG-581

[sarah-welton]

Please read the Contributing guide before making a PR.

• Read the Graphics repository & Yamato FAQ.

Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

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Purpose of this PR

Why is this PR needed, what hard problem is it solving/fixing?

Some documentation issues were identified when reviewing the first draft of the docs for 2021.2, per GSG-581:

• View Direction / View Vector
• Preview Mode Control
• Built-in Target

Updates related to Surface Gradient Workflows and the review of SpeedTree Subgraph Assets won't make the release cut-off - these will be addressed later.

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Testing status

Describe what manual/automated tests were performed for this PR

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Comments to reviewers

Notes for the reviewers you have assigned.

If I've added you as a reviewer, it's mostly just for visibility on what I already pinged you about over Slack :) Thanks!

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

Shader Graph
/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2
Depending on your PR, you may also want
/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_2021.2
/.yamato%252Fall-shadergraph_builtin_lighting.yml%2523PR_ShaderGraph_BuiltIn_Lighting_2021.2

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[jessebarker]

LGTM