[clang-tools-extra] [clang-doc][nfc] Avoid constructing SmallString in ToString method (PR #96921)

[Paul Kirth via cfe-commits]

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@@ -56,7 +56,7 @@ class HTMLTag {
   operator bool() = delete;
 
   bool IsSelfClosing() const;
-  llvm::SmallString<16> ToString() const;
+  const char* toString() const;
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ilovepi wrote:

Maybe this should be renamed to `c_str()` or `str()`?

https://github.com/llvm/llvm-project/pull/96921