multilib-fix: document why not to use __WORDSIZE
    
    Fedora's PackagingDrafts/MultilibTricks suggests the following
    c-header wrapper example:
    
      #include <bits/wordsize.h>
      #if __WORDSIZE == 32
      #include "config-32.h"
      #elif __WORDSIZE == 64
      #include "config-64.h"
      #else
      #error "Unknown word size"
      #endif
    
    Installing and using this wrapper is not really no-op, however; it
    has been proven to cause include-ordering issues.  So, even though
    the shorter stub works in most cases, document why we still prefer
    the longer #ifdef wrapper.  Thanks to Vít Ondruch for pointing
    this out.
    
    Related: rhbz#1412274