Fixed the update condition of cookie atlas and added doc for render texture cookies update.

[JulienIgnace-Unity]

Purpose of this PR

Fixed an issue with cookie texture atlas that would cause realtime textures to always update in the atlas in editor even when the content did not change.
Also added to the light documentation a section explaining what needs to be done for real time render texture cookies to update in the atlas.

This doc update is in response to this case: https://fogbugz.unity3d.com/f/cases/1259997/

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Testing status

Tested with the provided scene script (after removing recreation of render texture everyframe) and added a bool property to make sure that not calling IncrementUpdateCount would indeed stoped the atlas from updating, which was not the case before the fix. This also fixed the inconsistent behavior in the editor where texture would update every frame.

Manual Tests: What did you do?

• Opened test project + Run graphic tests locally
• Built a player
• Checked new UI names with UX convention
• Tested UI multi-edition + Undo/Redo + Prefab overrides + Alignment in Preset
• C# and shader warnings (supress shader cache to see them)
• Checked new resources path for the reloader (in developer mode, you have a button at end of resources that check the paths)

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[JordanL8]

Added some doc suggestions

[JordanL8]

HDRP allows you to use a RenderTexture as a light cookie. However, for the sake of performance, if you make any changes to the RenderTexture, HDRP does not automatically update the cookie atlas. To notify the system that the RenderTexture content has changed and thus make the system upload the change to the cookie atlas, call IncrementUpdateCount() on the RenderTexture. If you do not do this, the system does not update the cookie.