Closes #9574: Note that complex constructor doesn't allow whitespace around central operator.

Doc/library/functions.rst

@@ -247,6 +247,13 @@ are always available.  They are listed here in alphabetical order.
    the function serves as a numeric conversion function like :func:`int`
    and :func:`float`.  If both arguments are omitted, returns ``0j``.
 
+   .. note::
+
+      When converting from a string, the string must not contain whitespace
+      around the central ``+`` or ``-`` operator.  For example,
+      ``complex('1+2j')`` is fine, but ``complex('1 + 2j')`` raises
+      :exc:`ValueError`.
+
    The complex type is described in :ref:`typesnumeric`.