doc: new Buffer(size) gives no content guarantees

[phedny]

fixes #7230

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[tjfontaine]

can you refactor this to read something like: "Buffers behave differently than ArrayBuffers, their underlying memory is not initialized so the contents of the buffer after initialization is unknown. To initialize a buffer to empty use .fill(0)" or what have you

[sarathms]

@tjfontaine I've sent a pull request #9104 with the suggested rephrasing. Also without merge conflicts.

[trevnorris]

Closing this in favor of #9104