[clang] [llvm] [clang] Add/enhance documentation for some important classes. (PR #109795)

[Yanzuo Liu via llvm-commits]

================
@@ -79,8 +79,24 @@ enum class StringLiteralKind;
 // AST classes for statements.
 //===----------------------------------------------------------------------===//
 
-/// Stmt - This represents one statement.
+/// A statement or expression in the program.
 ///
+/// This is the base for the hierarchy of statements (ForStmt, ReturnStmt...)
+/// as well as expressions (Expr, CastExpr, IntegerLiteral...).
+/// Classing expressions as Stmt allows them to appear as statements without
+/// needing an extra "expression-statement" node.
+///
+/// Statements can have children and so form trees. e.g. `while (i>0) i--;`
+///
+///   WhileStmt
+///   |-BinaryOperator >
+///   | |-DeclRefExpr i
+///   | `-IntegerLiteral 0
+///   `-UnaryOperator --
+///       DeclRefExpr i
+///
----------------
zwuis wrote:

We can use `\code` and `\endcode` commands for code block so that it is treated as code instead of text by doxygen.

```cpp
/// \code
///   WhileStmt
///   |-BinaryOperator >
///   ...
/// \endcode
```

https://github.com/llvm/llvm-project/pull/109795