Document API change in ReScript 10.0.0

[harry-xm]

pages/docs/manual/latest/api/js/string-2.mdx and string.mdx should be updated to reflect breaking API change.

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In rescript@9.1.4, function signature of Js.String2.match_ is (Js.String2.t, Js.Re.t) => option<array<Js.String2.t>>.

In rescript@10.0.1, the function signature has one more nested option<> as follows.

(
  Js.String2.t,
  Js.Re.t,
) => option<array<option<Js.String2.t>>>

Copied from rescript-lang/rescript#5643.

[danielo515]

I was going crazy about this. Thanks for the bug report