Revise performance timing entries' names and descriptions

[ddbeck]

A follow up to #4237.

[tunetheweb]

Mostly LGTM but if we're capitalising some, then I'd capitalise them all. The specs mostly treat these as proper nouns (though not in all places I've noticed!).

And similarly I'd suggest adding "peformance entries" consistently.

I've also suggested changes to Visibility State and Resource Timing to make them clearer.

[tunetheweb]

Suggested change

	name: Element timing
	name: Element Timing performance entries

[tunetheweb]

Suggested change

	name: Layout instability
	name: Layout Instability performance entries

[tunetheweb]

Suggested change

	name: Navigation timing
	name: Navigation Timing performance entries

[tunetheweb]

Suggested change

	name: Paint timing
	name: Paint Timing performance entries

[tunetheweb]

Suggested change

	name: Resource timing
	name: Resource Timing performance entries

[tunetheweb]

Suggested change

	name: Soft navigation performance entries
	name: Soft Navigation performance entries

[tunetheweb]

Suggested change

	description: A `PerformanceResourceTiming` performance entry records when a network event happens while loading a resource, such as when a connection starts and ends. You can use this information to measure loading times.
	description: A `resource` performance entry records network event timings, such as when a connection starts and ends or loading time, for resources requested by a page.

[tunetheweb]

Suggested change

	name: Page visibility state
	name: Page Visibility State Entry performance entry

I think this is especially important to rename to differentiate it from page-visibility.yml (which is related but different - one just gets the current state, the other monitors for changes).