Add decimal docs to the core.

Doc/lib/lib.tex

@@ -120,6 +120,7 @@ and how to embed it in other applications.
 \input{libdoctest}
 \input{libunittest}
 \input{libtest}
+\input{libdecimal}
 \input{libmath}
 \input{libcmath}
 \input{librandom}