footnote on complex infinity

Author: skirpichev

peps/pep-0812.rst

@@ -368,7 +368,7 @@ For complex multiplication [8]_:
      z * w = w * z
      (-z) * w = -(z * w)
 
-- if one operand is an infinity and the other operand is a nonzero finite
+- if one operand is an infinity [9]_ and the other operand is a nonzero finite
   number or an infinity, then the result is an infinity.
 
 For division:
@@ -688,6 +688,11 @@ Footnotes
           >>> mul3(w, z)
           (inf+infj)
 
+.. [9] A complex value with at least one infinite part is regarded as an
+   infinity (even if its other part is a NaN).  A complex value is a finite
+   number if each of its parts is a finite number (neither infinite nor NaN).
+   A complex value is a zero if each of its parts is a zero.
+
 
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