version(StdDoc)

[Tony]

In std.compiler there is this code:

     /// Which vendor produced this compiler.
     version(StdDdoc)          Vendor vendor;
     else version(DigitalMars) Vendor vendor = Vendor.digitalMars;
     else version(GNU)         Vendor vendor = Vendor.gnu;
     else version(LDC)         Vendor vendor = Vendor.llvm;
     else version(D_NET)       Vendor vendor = Vendor.dotNET;
     else version(SDC)         Vendor vendor = Vendor.sdc;
     else                      Vendor vendor = Vendor.unknown;

What is the situation in which the identifier StdDoc is set?