[MLIR][Doc] Also print summarys for passes on a newline

This patch is improves upon https://reviews.llvm.org/D152621. There, I pointed out some issues with D152621, which I'll repeat here.

> Passes use a different logic for generating the documentation; which I didn't update to be in-line with this change.

Fixed by defining and using `mlir::tblgen::emitSummary`. This is now used in `OpDocGen.cpp` and `PassDocGen.cpp`.

Note that the passes documentation currently prints the summary behind the pass argument. For example:

```
#### -arm-neon-2d-to-intr: Convert Arm NEON structured ops to intrinsics
```
at https://mlir.llvm.org/docs/Passes/#-promote-buffers-to-stack-promotes-heap-based-allocations-to-automatically-managed-stack-based-allocations.

This currently differs from how the summary is printed for Ops. For example:

```
#### amdgpu.lds_barrier (::mlir::amdgpu::LDSBarrierOp) ¶

**Summary:** _Barrier that includes a wait for LDS memory operations._
```

at https://mlir.llvm.org/docs/Dialects/AMDGPU/#amdgpulds_barrier-mliramdgpuldsbarrierop.

The changes in this patch ensure that:

1. The summary is always printed on a new line.
2. The summary is always printed in italic.
3. The summary always starts with a capital letter.

I've dropped the `**Summary:**`, which was introduced in D152621, because only italicization should be already clear enough.

> `amx.tdpbssd` shows **Summary:** __ meaning that apparently hasSummary does not guarantee a non-empty summary.

This is fixed by double-checking `!summary.empty()`, because the following code

```cpp
void mlir::tblgen::emitSummary(StringRef summary, raw_ostream &os) {
  if (!summary.empty()) {
    char first = std::toupper(summary.front());
    llvm::StringRef rest = summary.drop_front();
    os << "\n_" << first << rest << "_\n\n";
  } else {
    os << "\n_" << "foo" << "_\n\n";
  }
}
```
generates the following Markdown:
```
### `amx.tdpbssd` (::mlir::amx::x86_amx_tdpbssd)

_foo_
```
in `tools/mlir/docs/Dialects/AMX.md`.

> Summary fields containing * cancel the italicization, so the * should probably be escaped to solve this. EDIT: Nope. This is because mlir-www runs Hugo 0.80 whereas 0.111 correctly parses _Raw Buffer Floating-point Atomic Add (MI-* only)_ as an italicized string.

This will be fixed by llvm/mlir-www#152.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D152648

Author: rikhuijzer

mlir/docs/DefiningDialects/Operations.md

@@ -165,9 +165,10 @@ let description = [{
 Placing the documentation at the beginning is recommended since it helps in
 understanding the operation.
 
-> *   Place documentation at the beginning of the operation definition
-> *   The summary should be short and concise. It should be a one-liner without
->     trailing punctuation. Put expanded explanation in description.
+> *   Place documentation at the beginning of the operation definition.
+> *   The summary should be short and concise. It should be a one-liner
+>     starting with a capital letter and without trailing punctuation.
+>     Put expanded explanation in the description.
 
 ### Operation arguments
 

mlir/tools/mlir-tblgen/DocGenUtilities.h

@@ -23,6 +23,10 @@ class raw_ostream;
 namespace mlir {
 namespace tblgen {
 
+// Emit the summary. To avoid confusion, the summary is styled differently from
+// the description.
+void emitSummary(llvm::StringRef summary, llvm::raw_ostream &os);
+
 // Emit the description by aligning the text to the left per line (e.g.
 // removing the minimum indentation across the block).
 //

mlir/tools/mlir-tblgen/OpDocGen.cpp

@@ -47,6 +47,14 @@ using namespace mlir;
 using namespace mlir::tblgen;
 using mlir::tblgen::Operator;
 
+void mlir::tblgen::emitSummary(StringRef summary, raw_ostream &os) {
+  if (!summary.empty()) {
+    char first = std::toupper(summary.front());
+    llvm::StringRef rest = summary.drop_front();
+    os << "\n_" << first << rest << "_\n\n";
+  }
+}
+
 // Emit the description by aligning the text to the left per line (e.g.,
 // removing the minimum indentation across the block).
 //
@@ -172,7 +180,7 @@ static void emitOpDoc(const Operator &op, raw_ostream &os) {
 
   // Emit the summary, syntax, and description if present.
   if (op.hasSummary())
-    os << "\n**Summary:** _" << op.getSummary() << "_\n\n";
+    emitSummary(op.getSummary(), os);
   if (op.hasAssemblyFormat())
     emitAssemblyFormat(op.getOperationName(), op.getAssemblyFormat().trim(),
                        os);

mlir/tools/mlir-tblgen/PassDocGen.cpp

@@ -21,8 +21,8 @@ using namespace mlir::tblgen;
 
 /// Emit the documentation for the given pass.
 static void emitDoc(const Pass &pass, raw_ostream &os) {
-  os << llvm::formatv("### `-{0}`: {1}\n", pass.getArgument(),
-                      pass.getSummary());
+  os << llvm::formatv("### `-{0}`\n", pass.getArgument());
+  emitSummary(pass.getSummary(), os);
   emitDescription(pass.getDescription(), os);
 
   // Handle the options of the pass.