[mlir][sparse] updated setter/getter comments

For the getters, it is bad practice to keep the reference
around for too long, as explained in the new comment

Reviewed By: gussmith23

Differential Revision: https://reviews.llvm.org/D105599

mlir/include/mlir/Dialect/SparseTensor/Utils/Merger.h

@@ -168,10 +168,13 @@ class Merger {
   /// Returns true if any set bit corresponds to queried dim.
   bool hasAnyDimOf(const llvm::BitVector &bits, Dim d) const;
 
-  /// Setter
+  /// Dimension setter.
   void setDim(unsigned t, unsigned i, Dim d) { dims[t][i] = d; }
 
-  /// Getters.
+  /// Convenience getters to immediately access the stored nodes.
+  /// Typically it is inadvisible to keep the reference around, as in
+  /// "TensorExpr &te = merger.exp(e))", since insertions into the merger
+  /// may cause data movement and invalidate the underlying memory address.
   TensorExp &exp(unsigned e) { return tensorExps[e]; }
   LatPoint &lat(unsigned l) { return latPoints[l]; }
   SmallVector<unsigned, 16> &set(unsigned s) { return latSets[s]; }