gh-141510: Document isinstance change for frozendict in What's New (#… #2235

Job	Run time
x86_64-pc-windows-msvc/msvc-free-threading	4m 58s
x86_64-pc-windows-msvc/msvc	10m 33s
aarch64-apple-darwin/clang	9m 55s
x86_64-apple-darwin/clang	16m 7s
x86_64-unknown-linux-gnu/gcc	9m 50s
aarch64-unknown-linux-gnu/gcc	7m 21s
x86_64-unknown-linux-gnu/gcc-free-threading	8m 45s
	1h 7m 29s

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