gh-143674: document complex representation using f/d formats in struct

Doc/library/struct.rst

@@ -280,6 +280,10 @@ platform-dependent.
 .. versionchanged:: 3.14
    Added support for the ``'F'`` and ``'D'`` formats.
 
+Complex numbers may also be represented using two consecutive floating-point
+format characters. In this case, the real part is stored first, followed by
+the imaginary part. The format characters ``f`` and ``d`` determine the type
+used for each component.
 
 Notes:
 

Modules/_struct.c

@@ -2688,7 +2688,9 @@ these can be preceded by a decimal repeat count:\n\
   x: pad byte (no data); c:char; b:signed byte; B:unsigned byte;\n\
   ?: _Bool (requires C99; if not available, char is used instead)\n\
   h:short; H:unsigned short; i:int; I:unsigned int;\n\
-  l:long; L:unsigned long; f:float; d:double; e:half-float.\n\
+    l:long; L:unsigned long; f:float; d:double; e:half-float.\n\
+  For complex numbers, the real and imaginary parts are represented by two\n\
+  consecutive values of the specified floating-point type.\n\
 Special cases (preceding decimal count indicates length):\n\
   s:string (array of char); p: pascal string (with count byte).\n\
 Special cases (only available in native format):\n\