Update Compute Pressure API documentation

[Ansh101112]

Fixes #6284

Changes proposed in this pull request:

• Remove the "static" keyword from the method description.
• Correctly describe the method as a read-only function.
• Reference the relevant specification.

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:white_check_mark: Preview (static) for commit 89040631e5a5ec42b2fdf47ba90d7669bb970e81 available at https://pr-7745-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-7745-static-dot-dcc-staging.uc.r.appspot.com/blog/compute-pressure-origin-trial-2/
• https://pr-7745-static-dot-dcc-staging.uc.r.appspot.com/docs/web-platform/compute-pressure/