version(StdDoc)

[Tony via Digitalmars-d-learn]

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?