Add more documentation to the linear solver enums.

Change-Id: Id57f76f73fa38043c0b6729972b1de8578ad7ede

internal/ceres/linear_solver.h

diff --git a/internal/ceres/linear_solver.h b/internal/ceres/linear_solver.h
index cb26356..8737c7c 100644
--- a/internal/ceres/linear_solver.h
+++ b/internal/ceres/linear_solver.h
@@ -62,7 +62,10 @@
   // the linear system being poorly conditioned.
   LINEAR_SOLVER_FAILURE,
 
-  // Solver failed with a fatal error that cannot be recovered from.
+  // Solver failed with a fatal error that cannot be recovered from,
+  // e.g. CHOLMOD ran out of memory when computing the symbolic or
+  // numeric factorization or an underlying library was called with
+  // the wrong arguments.
   LINEAR_SOLVER_FATAL_ERROR
 };