Fix emitting attribute documentation

[marbre]

Summary

This fixes the documentation emitted for type parameters. Also adds a
missing empty line, rendered as line break in mark down.

Co-authored-by: Simon Camphausen <[e-mail]>

[rriddle]

Can you show an example of before/after?

[marbre]

In D97267#2581276, @rriddle wrote:

Can you show an example of before/after?

Sure! For EmitC.md, the resulting doc would change as in the following patch:

--- ../docs/EmitC.md    2021-02-17 09:18:41.760151054 +0100
+++ docs/Dialects/EmitC.md      2021-02-23 09:05:03.999213035 +0100
@@ -233,11 +233,12 @@ yielded.
 ### `OpaqueType` (OpaqueType)
 
 An opaque type
+
 An opaque data type of which the value gets emitted as is.
 
 #### Type parameters:
 
 | Parameter | C++ type | Description |
 | :-------: | :-------: | ----------- |
-| value | `OpaqueType` | the opaque value |
+| value | `::llvm::StringRef` | the opaque value |

[rriddle]

Nice, that looks much better.

mlir/tools/mlir-tblgen/OpDocGen.cpp

static void emitTypeDefDoc(TypeDef td, raw_ostream &os) {
   os << llvm::formatv("### `{0}` ({1})\n", td.getName(), td.getCppClassName());
 
   // Emit the summary, syntax, and description if present.
   if (td.hasSummary())
     os << "\n" << td.getSummary() << "\n";
   if (td.getMnemonic() && td.getPrinterCode() && *td.getPrinterCode() == "" &&
       td.getParserCode() && *td.getParserCode() == "")
     emitTypeAssemblyFormat(td, os);
-  if (td.hasDescription())
+  if (td.hasDescription()) {
+    os << "\n";
     mlir::tblgen::emitDescription(td.getDescription(), os);
+  }
 
   // Emit attribute documentation.
   SmallVector<TypeParameter, 4> parameters;
   td.getParameters(parameters);
   if (!parameters.empty()) {
     os << "\n#### Type parameters:\n\n";
     os << "| Parameter | C++ type | Description |\n"
        << "| :-------: | :-------: | ----------- |\n";
     for (const auto &it : parameters) {
       auto desc = it.getSummary();
-      os << "| " << it.getName() << " | `" << td.getCppClassName() << "` | "
+      os << "| " << it.getName() << " | `" << it.getCppType() << "` | "
          << (desc ? *desc : "") << " |\n";
     }
   }
 
   os << "\n";
 }
 
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