ReseedingRng: fix doc comment on reseeding behavior after UNIX fork #566

Job	Run time
test-ios	37s
Check doc	1m 47s
test-cross (ubuntu-latest, mips-unknown-linux-gnu, stable)	2m 3s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 41s
test (macos-latest, x86_64-apple-darwin, stable)	3m 16s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 39s
test (windows-latest, x86_64-pc-windows-msvc, beta)	2m 31s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.56.0)	3m 38s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 11s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	41s
test-miri	4m 22s
test-no-std	17s
	24m 43s

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