Doc: add a not that `export *` is only possible once per file #13762

Job	Run time
cpp_test_driver (ubuntu-22.04)	5m 21s
cpp_test_driver (macos-12)	5m 49s
ffi_32bit_build	1m 16s
cpp_test_driver (windows-2022)	6m 22s
build_and_test (ubuntu-22.04, stable)	8m 42s
build_and_test (ubuntu-22.04, 1.70)	10m 50s
mcu (pico-st7789, thumbv6m-none-eabi)	34s
cpp_cmake (ubuntu-22.04)	5m 53s
mcu_esp	2m 29s
build_and_test (macos-11, stable)	32m 3s
mcu (stm32h735g, thumbv7em-none-eabihf)	38s
build_and_test (windows-2022, stable)	8m 26s
build_and_test (windows-2022, 1.70)	7m 55s
node_test (ubuntu-22.04)	2m 50s
cpp_cmake (macos-12)	12m 38s
build_and_test (windows-2022, beta, --exclude ffmpeg --exclude gstreamer-player)	7m 38s
python_test (ubuntu-22.04)	1m 57s
node_test (macos-11)	17m 1s
python_test (macos-11)	2m 2s
cpp_cmake (windows-2022)	7m 45s
node_test (windows-2022)	15m 16s
python_test (windows-2022)	1m 30s
uefi-demo	29s
android	1m 17s
esp-idf	25m 39s
fmt_test	1m 30s
tree-sitter	9s
updater_test (0.3.0)	1m 28s
docs / docs	6m 17s
wasm / wasm	3m 38s
wasm_demo / wasm_demo	13m 14s
wasm / lint-slintpad	26s
wasm / build-slintpad	54s
cpp_package_test	2m 36s
	3h 42m 32s

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