[docs] Add documentation to note "-gb" Prestissimo Configs are using GiB units and not gB units

[minhancao]

Description

Add documentation to note "-gb" Prestissimo Configs are using GiB units and not gB units

Motivation and Context

Resolves #24125
"-gb" Prestissimo configs like system-memory-gb are actually using GiB units and not gB units. This documentation will help clarify any confusion from users.

Impact

Test Plan

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Release Notes

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== RELEASE NOTES ==

Documentation
* Add documentation to note "-gb" Prestissimo Configs are using GiB units and not gB units.

[steveburnett]

Thanks for the doc! One question about the value.

[steveburnett]

LGTM! (docs)

Pull updated branch, new local doc build, looks good. Thanks for adding this to the doc!