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.
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
?: _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:float complex; D:double complex.\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\