Add minor doc improvements (#70984)

- Document legacy ops lowered by `convert-func-to-llvm` and link it to a
GH issue
- Document the need for bufferization before `convert-linalg-to-loops`
- Document the ops lowered by `expand-strided-metadata`.

mlir/include/mlir/Conversion/Passes.td

@@ -391,6 +391,10 @@ def ConvertFuncToLLVMPass : Pass<"convert-func-to-llvm", "ModuleOp"> {
     1 value is returned, packed into an LLVM IR struct type. Function calls and
     returns are updated accordingly. Block argument types are updated to use
     LLVM IR types.
+
+    Note that until https://github.com/llvm/llvm-project/issues/70982 is resolved,
+    this pass includes patterns that lower `arith` and `cf` to LLVM. This is legacy
+    code due to when they were all converted in the same pass.
   }];
   let dependentDialects = ["LLVM::LLVMDialect"];
   let options = [

mlir/include/mlir/Dialect/Linalg/Passes.td

@@ -69,6 +69,13 @@ def LinalgLowerToAffineLoops : Pass<"convert-linalg-to-affine-loops"> {
 
 def LinalgLowerToLoops : Pass<"convert-linalg-to-loops"> {
   let summary = "Lower the operations from the linalg dialect into loops";
+  let description = [{
+    Lowers the `linalg` ops to loop nests using `scf.for`.
+
+    Pre-condition: the operands used by the `linalg` ops have buffer semantics,
+    i.e., tensor operands and results must be converted to memrefs via
+    bufferization.
+  }];
   let constructor = "mlir::createConvertLinalgToLoopsPass()";
   let dependentDialects = [
     "linalg::LinalgDialect",

mlir/include/mlir/Dialect/MemRef/Transforms/Passes.td

@@ -196,6 +196,14 @@ def ExpandStridedMetadata : Pass<"expand-strided-metadata"> {
     In particular, this pass transforms operations into explicit sequence of
     operations that model the effect of this operation on the different metadata.
     This pass uses affine constructs to materialize these effects.
+
+    Supported ops include:
+
+    - `memref.collapse_shape`
+    - `memref.expand_shape`
+    - `memref.extract_aligned_pointer_as_index`
+    - `memref.extract_strided_metadata`
+    - `memref.subview`
   }];
   let constructor = "mlir::memref::createExpandStridedMetadataPass()";
   let dependentDialects = [

mlir/lib/Conversion/FuncToLLVM/FuncToLLVM.cpp

@@ -804,7 +804,8 @@ struct ConvertFuncToLLVMPass
     RewritePatternSet patterns(&getContext());
     populateFuncToLLVMConversionPatterns(typeConverter, patterns, symbolTable);
 
-    // TODO: Remove these in favor of their dedicated conversion passes.
+    // TODO(https://github.com/llvm/llvm-project/issues/70982): Remove these in
+    // favor of their dedicated conversion passes.
     arith::populateArithToLLVMConversionPatterns(typeConverter, patterns);
     cf::populateControlFlowToLLVMConversionPatterns(typeConverter, patterns);