RFR: 8261023: Document why memory pretouch must be a store [v2]

[Thomas Schatzl]

On Wed, 3 Feb 2021 11:29:40 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   shade review
>
> Marked as reviewed by shade (Reviewer).

Thanks @shipilev @walulyai for your reviews.

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PR: https://git.openjdk.java.net/jdk/pull/2373