[clang] [clang][docs] Document single-vector __builtin_shufflevector form (PR #214698)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: richardmilles

<details>
<summary>Changes</summary>

Document that `__builtin_shufflevector` also accepts a single-vector form that only lists indices.

Fixes #<!-- -->59678


---
Full diff: https://github.com/llvm/llvm-project/pull/214698.diff


1 Files Affected:

- (modified) clang/docs/LanguageExtensions.md (+2) 


``````````diff
diff --git a/clang/docs/LanguageExtensions.md b/clang/docs/LanguageExtensions.md
index f5313e0378ca0..cda8d27875c42 100644
--- a/clang/docs/LanguageExtensions.md
+++ b/clang/docs/LanguageExtensions.md
@@ -3521,6 +3521,8 @@ for the implementation of various target-specific header files like
 
 ```c++
 __builtin_shufflevector(vec1, vec2, index1, index2, ...)
+
+Clang also accepts a single-vector form `__builtin_shufflevector(vec, index1, index2, ...)` that shuffles lanes within `vec` only.
 ```
 
 **Examples**:

``````````

</details>


https://github.com/llvm/llvm-project/pull/214698