Add some documentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25150 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/System/Program.h

@@ -50,6 +50,8 @@ namespace sys {
       /// executed. It is presumed this is the result of the FindProgramByName
       /// method.
       /// @returns an integer result code indicating the status of the program.
+      /// A zero or positive value indicates the result code of the program. A
+      /// negative value is the signal number on which it terminated.
       /// @throws std::string on a variety of error conditions or if the invoked
       /// program aborted abnormally.
       /// @see FindProgrambyName