doc/go1.21: mention changes to the reflect package
Added Value.Clear, deprecated SliceHeader and StringHeader.
For #55002
For #56906
Change-Id: Ib7497aff830d56fad90c31ec28596e71a448e9ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/498757 Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
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