simlib: Add `$buf` disclaimer #814

Job	Run time
pre_job	5s
test-compile (ubuntu-latest, clang-14)	11m 11s
test-compile (ubuntu-latest, gcc-10)	11m 9s
test-compile (ubuntu-latest, clang)	21m 27s
test-compile (ubuntu-latest, gcc)	23m 3s
test-compile (macos-13, clang)	31m 46s
test-compile (ubuntu-20.04, clang-10)	11m 48s
	1h 50m 29s

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