gh-143674: Document F/D complex format characters in struct module

Add documentation for the F (complex from two floats) and D (complex
from two doubles) format characters in the struct module docstring.
These format characters were implemented but not documented.

Author: moneebullah25

Modules/_struct.c

@@ -2689,6 +2689,7 @@ these can be preceded by a decimal repeat count:\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\
+  F:complex (two floats); D:complex (two doubles).\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\