docs: clarify that BODY_SIZE_LIMIT is numeric and does not accept '512kb'

[bluepuma77]

The current text "Defaults to 512kb" implies, that this is the way to set the variable. But it needs to be numeric, no unit.

Note: first pull request ever 🙂

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[benmccann]

Note that if we moved forward with #11680 we would no longer need this PR

[benmccann]

I merged #11680, so closing this PR