Bug 1979385 [wpt PR 53981] - Add step-by-step instructions to cache usage partitioning WPT, a=testonly

Automatic update from web-platform-tests
Add step-by-step instructions to cache usage partitioning WPT

This is a maintenance CL to improve the readability of the WPT
which demonstrates the impact of partitioning on cache usage data.

Change-Id: I0b0aee658ff55790ee83050d0d25c92b640fef08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6781574
Commit-Queue: Kyra Seevers <kyraseeverschromium.org>
Reviewed-by: Andrew Williams <awilliachromium.org>
Cr-Commit-Position: refs/heads/main{#1492090}

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wpt-commits: 391f23409fe487ad4dd31669edc6905e183bf27c
wpt-pr: 53981

UltraBlame original commit: 107477c4fb1d2220afaea26c38c618376a29129e

Author: marco-c