[clang] aligned attribute doc (PR #195278)

[Aaron Ballman via cfe-commits]

================
@@ -10000,3 +9983,46 @@ different languages to coexist on the same call stack while each interpreting
 exceptions according to their own rules.
   }];
 }
+
+def AlignedDocs : Documentation {
+  let Category = DocCatDecl;
+  let Heading = "aligned";
+  let Content = [{
+The ``aligned`` attribute specifies a minimum alignment (in bytes) for a 
+variable, structure field, or type definition. The attribute takes an 
+optional non-negative integral constant expression. If the value is a 
+power of two, the compiler ensures the declaration is aligned to at least 
+that boundary; a value of zero has no effect.
+
+.. code-block:: c++
+
+  struct __attribute__((aligned(8))) S { 
+    short f[3]; 
+  };
+
+If the alignment factor is omitted, the compiler defaults to the largest(the most strict) 
+alignment supported by the target machine for any data type. 
+
+Applying this attribute to a structure or union changes the type's layout 
----------------
AaronBallman wrote:

```suggestion
Applying this attribute to a structure or union, or to one of its members, changes the type's layout 
```
(may need to re-flow to the usual 80 column limits.)

https://github.com/llvm/llvm-project/pull/195278