[mlir][vector] Improve vector.gather description

mlir/include/mlir/Dialect/Vector/IR/VectorOps.td

@@ -2058,32 +2058,36 @@ def Vector_GatherOp :
     Results<(outs AnyVectorOfNonZeroRank:$result)> {
 
   let summary = [{
-    gathers elements from memory or ranked tensor into a vector as defined by an
-    index vector and a mask vector
+    Gathers elements from memory or ranked tensor into a vector as defined by an
+    index vector and a mask vector.
   }];
 
   let description = [{
     The gather operation returns an n-D vector whose elements are either loaded
-    from memory or ranked tensor, or taken from a pass-through vector, depending
+    from a k-D memref or tensor, or taken from an n-D pass-through vector, depending
     on the values of an n-D mask vector.
-    If a mask bit is set, the corresponding result element is defined by the base
-    with indices and the n-D index vector (each index is a 1-D offset on the base).
-    Otherwise, the corresponding element is taken from the n-D pass-through vector.
-    Informally the semantics are:
+
+    If a mask bit is set, the corresponding result element is taken from `base`
+    at an index defined by k `indices` and n-D `index_vec`. Otherwise, the element
+    is taken from the pass-through vector. As an example, suppose that base is
+    3-D and the result is 2-D. The indexing semantics are then,
+
     ```
-    result[0] := if mask[0] then base[index[0]] else pass_thru[0]
-    result[1] := if mask[1] then base[index[1]] else pass_thru[1]
-    etc.
+    result[i,j] := if mask[i,j] then
+                      base[indices[0], indices[1], indices[2] + index_vec[i,j]]
+                   else
+                      pass_thru[i,j]
     ```
+    The index into `base` only varies in the dimension k-1.
 
     If a mask bit is set and the corresponding index is out-of-bounds for the
     given base, the behavior is undefined. If a mask bit is not set, the value
     comes from the pass-through vector regardless of the index, and the index is
     allowed to be out-of-bounds.
 
     The gather operation can be used directly where applicable, or can be used
-    during progressively lowering to bring other memory operations closer to
-    hardware ISA support for a gather.
+    during progressive lowering to bring other memory operations closer to hardware
+    ISA support for a gather.
 
     Examples: