X509_dup.pod: add caveat that extra data is not copied and hints, e.g… #1017

Job	Run time
compiler (gcc-7, ubuntu-20.04)	3m 53s
compiler (gcc-8, ubuntu-20.04)	4m 1s
compiler (gcc-9, ubuntu-20.04)	4m 5s
compiler (gcc-10, ubuntu-20.04)	3m 44s
compiler (gcc-11, ubuntu-22.04)	4m 14s
compiler (gcc-12, ubuntu-22.04)	4m 32s
compiler (gcc-13, ubuntu-22.04, ubuntu-toolchain-r/test)	4m 19s
compiler (clang-6.0, ubuntu-20.04)	4m 15s
compiler (clang-7, ubuntu-20.04)	4m 11s
compiler (clang-8, ubuntu-20.04)	4m 30s
compiler (clang-9, ubuntu-20.04)	4m 33s
compiler (clang-10, ubuntu-20.04)	4m 22s
compiler (clang-11, ubuntu-20.04)	4m 10s
compiler (clang-12, ubuntu-20.04)	4m 10s
compiler (clang-13, ubuntu-22.04)	4m 33s
compiler (clang-14, ubuntu-22.04)	4m 39s
compiler (clang-15, ubuntu-22.04, jammy)	4m 40s
compiler (clang-16, ubuntu-22.04, jammy)	5m 6s
compiler (clang-17, ubuntu-22.04, jammy)	5m 12s
	1h 23m 9s

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