Add more details on the outputs JSON array to the docs.

Author: bd1976bris

llvm/docs/DTLTO.rst

@@ -138,6 +138,14 @@ Reserved Entries:
 - The first entry in each job's ``outputs`` array is the primary output object
   file.
 
+For the ``outputs`` array, only the first entry is reserved for the primary
+output file; there is no guaranteed order for the remaining entries. The primary
+output file is specified in a reserved entry because some distribution systems
+rely on this path - for example, to provide a meaningful user label for
+compilation jobs. Initially, the DTLTO implementation will not produce more than
+one output file. However, in the future, if LTO options are added that imply
+additional output files, those files will also be included in this array.
+
 Command-line arguments and input/output files are stored separately to allow
 the remote compiler to be changed without updating the distributors, as the
 distributors do not need to understand the details of the compiler command