complex: fix comment in cacosh

The principal expressions defining acosh and acos are such that

  acosh(z) = ±i acos(z)

where the + is only true on the Im(z)>0 half of the complex plane
(and partly on Im(z)==0 depending on number representation).

fix the comment without expanding on the details.

src/complex/cacosh.c

@@ -1,6 +1,6 @@
 #include "complex_impl.h"
 
-/* acosh(z) = i acos(z) */
+/* acosh(z) = ±i acos(z) */
 
 double complex cacosh(double complex z)
 {