[clang] [Clang][analyzer] add documentation for optin performance padding (padding checker) #73675 (PR #86411)

[Balazs Benics via cfe-commits]

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@@ -804,10 +804,38 @@ Check for performance anti-patterns when using Grand Central Dispatch.
 
 .. _optin-performance-Padding:
 
-optin.performance.Padding
-"""""""""""""""""""""""""
+optin.performance.Padding (C, C++, ObjC)
+""""""""""""""""""""""""""""""""""""""""
 Check for excessively padded structs.
 
+This checker detects structs with excessive padding, which can lead to wasted memory and decreased performance. Padding bytes are added by compilers to align data within the struct for performance optimization or memory alignment purposes. However, excessive padding can significantly increase the size of the struct without adding useful data, leading to inefficient memory usage, cache misses, and decreased performance.
+
+.. code-block:: c
+
+   #include <stdio.h>
+   // #pragma pack(1) // Uncomment it to disable structure padding
+   struct TestStruct {
+       char data1;  // 1 byte
+       char data2;  // 1 byte
+       int data3;   // 4 bytes
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steakhal wrote:

We usually indent code by 2 spaces in this `checkers.rst` file. Same for the `main` function body.
Also avoid trailing spaces in the empty lines.

https://github.com/llvm/llvm-project/pull/86411