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Thanks for reporting this.
Rather than documenting corner cases explicitly, maybe it would be enough to point to the C99 standard: the corner cases for the math functions should (modulo bugs) all follow Annex F of the C standard, while the corner cases for the cmath functions follow Annex G. This doesn't quite tell the complete story, because some functions (for example the cmath.rect function) don't appear in the C standards, but it would avoid cluttering the function descriptions with special cases.
Certainly the "All functions ..." sentence is inaccurate, and should be removed or revised. I believe it's true for all single-argument functions, though.