[clang-tools-extra] [clang-tidy] Emit warnings from user headers by default (PR #164165)

[Baranov Victor via cfe-commits]

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@@ -133,13 +133,15 @@ file, if any.
 static cl::opt<std::string> HeaderFilter("header-filter", desc(R"(
 Regular expression matching the names of the
 headers to output diagnostics from. Diagnostics
-from the main file of each translation unit are
-always displayed.
+from the main file and all non-system headers 
+of each translation unit are always displayed.
+Set this option to an empty string to disable 
+diagnostics from non-system headers.
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vbvictor wrote:

> I would simply keep the original text as-is and document what the default value for this option is (if it's not already stated somewhere else).

I think the best place to document the option is in "help" message, so I reworked it a and separated sentences for main file and non-system headers

https://github.com/llvm/llvm-project/pull/164165