Update struct docstring, slashing most of it

The struct docstring is much lengthier than most other docstrings.
The online documentation is much more complete.
Fix this docstring by removing most of it, referring instead to the online documentation.

Author: michael-the1

Modules/_struct.c

@@ -2874,9 +2874,9 @@ in the C struct.\n\
 \n\
 The optional first format char indicates byte order, size and alignment:\n\
   @: native order, size & alignment (default)\n\
-  =: native order, std. size & alignment\n\
-  <: little-endian, std. size & alignment\n\
-  >: big-endian, std. size & alignment\n\
+  =: native order, std. size, no alignment\n\
+  <: little-endian, std. size, no alignment\n\
+  >: big-endian, std. size, no alignment\n\
   !: same as >\n\
 \n\
 The remaining characters indicate types of args and must match exactly;\n\