[MLIR][NFC] Fix mix up between dialect attribute values and names

Clear up documentation on dialect attribute values. Fix/improve
ModuleOp verifier error message on dialect prefixed attribute names.
Additional discussion is here:
https://llvm.discourse.group/t/moduleop-attributes/2325

Differential Revision: https://reviews.llvm.org/D92502

mlir/docs/LangRef.md

@@ -1369,39 +1369,39 @@ Example:
 
 Similarly to operations, dialects may define custom attribute values. The
 syntactic structure of these values is identical to custom dialect type values,
-except that dialect attributes values are distinguished with a leading '#',
-while dialect types are distinguished with a leading '!'.
+except that dialect attribute values are distinguished with a leading '#', while
+dialect types are distinguished with a leading '!'.
 
 ```
-dialect-attribute ::= '#' opaque-dialect-item
-dialect-attribute ::= '#' pretty-dialect-item
+dialect-attribute-value ::= '#' opaque-dialect-item
+dialect-attribute-value ::= '#' pretty-dialect-item
 ```
 
-Dialect attributes can be specified in a verbose form, e.g. like this:
+Dialect attribute values can be specified in a verbose form, e.g. like this:
 
 ```mlir
-// Complex attribute
+// Complex attribute value.
 #foo<"something<abcd>">
 
-// Even more complex attribute
+// Even more complex attribute value.
 #foo<"something<a%%123^^^>>>">
 ```
 
-Dialect attributes that are simple enough can use the pretty format, which is a
-lighter weight syntax that is equivalent to the above forms:
+Dialect attribute values that are simple enough can use the pretty format, which
+is a lighter weight syntax that is equivalent to the above forms:
 
 ```mlir
 // Complex attribute
 #foo.something<abcd>
 ```
 
-Sufficiently complex dialect attributes are required to use the verbose form for
-generality. For example, the more complex type shown above wouldn't be valid in
-the lighter syntax: `#foo.something<a%%123^^^>>>` because it contains characters
-that are not allowed in the lighter syntax, as well as unbalanced `<>`
-characters.
+Sufficiently complex dialect attribute values are required to use the verbose
+form for generality. For example, the more complex type shown above would not be
+valid in the lighter syntax: `#foo.something<a%%123^^^>>>` because it contains
+characters that are not allowed in the lighter syntax, as well as unbalanced
+`<>` characters.
 
-See [here](Tutorials/DefiningAttributesAndTypes.md) to learn how to define dialect
+See [here](Tutorials/DefiningAttributesAndTypes.md) on how to define dialect
 attribute values.
 
 ### Standard Attribute Values

mlir/lib/IR/BuiltinDialect.cpp

@@ -228,9 +228,9 @@ static LogicalResult verify(ModuleOp op) {
             ArrayRef<StringRef>{mlir::SymbolTable::getSymbolAttrName(),
                                 mlir::SymbolTable::getVisibilityAttrName()},
             attr.first.strref()))
-      return op.emitOpError()
-             << "can only contain dialect-specific attributes, found: '"
-             << attr.first << "'";
+      return op.emitOpError() << "can only contain attributes with "
+                                 "dialect-prefixed names, found: '"
+                              << attr.first << "'";
   }
 
   return success();

mlir/test/IR/invalid-module-op.mlir

@@ -44,7 +44,7 @@ func @module_op() {
 
 // -----
 
-// expected-error@+1 {{can only contain dialect-specific attributes}}
+// expected-error@+1 {{can only contain attributes with dialect-prefixed names}}
 module attributes {attr} {
 }