doc/go1.21: mention regexp.MarshalText and UnmarshalText
For #46159
Change-Id: Ia9cc0827a89d362532d1a662b791de8eebbfb2fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/498756 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Eli Bendersky <eliben@google.com>
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