Clarifying behavior for Crypto.PublicKey.RSA.export_key in docs #850

Job	Run time
mypy	22s
test_c_linux (0, x64)	49s
build_python_27 (ubuntu-20.04, yes)	2m 30s
test_c_linux (0, x32)	59s
build_python_27 (ubuntu-20.04, no)	2m 23s
test_c_linux (1, x64)	52s
build (3.5, yes, ubuntu-20.04)	2m 20s
build_python_27 (windows-latest, yes)	3m 57s
test_c_windows (x64)	51s
build (3.5, no, ubuntu-20.04)	2m 10s
test_c_linux (1, x32)	1m 6s
build_python_27 (windows-latest, no)	3m 45s
test_c_windows (win32)	52s
build (3.6, yes, ubuntu-20.04)	2m 26s
build (3.6, no, ubuntu-20.04)	2m 16s
build (3.7, yes, ubuntu-20.04)	2m 20s
build (3.7, no, ubuntu-20.04)	2m 18s
build (3.8, yes, ubuntu-20.04)	2m 8s
build (3.8, no, ubuntu-20.04)	2m 6s
build (3.9, yes, ubuntu-20.04)	2m 3s
build (3.9, no, ubuntu-20.04)	2m 7s
build (3.10, yes, ubuntu-20.04)	2m 0s
build (3.10, no, ubuntu-20.04)	2m 5s
build (3.11, yes, ubuntu-20.04)	1m 46s
build (3.11, no, ubuntu-20.04)	1m 58s
build (3.12-dev, yes, ubuntu-20.04)	1m 54s
build (3.12-dev, no, ubuntu-20.04)	1m 55s
build (3.11, yes, macos-12)	3m 50s
build (3.11, no, windows-latest)	3m 54s
build (3.11, yes, windows-latest)	4m 8s
build (3.12-dev, no, windows-latest)	4m 7s
build (3.12-dev, yes, windows-latest)	4m 23s
build (pypy2.7, no, ubuntu-latest)	1m 28s
build (pypy3.9, no, ubuntu-latest)	1m 39s
	1h 15m 47s

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