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

[Victor Chernyakin 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.
----------------
localspook wrote:

This wording is a bit confusing to me. Warnings from non-system headers are *not* always displayed, the point is that you can disable them with this option, right? Something like this seems more accurate to me:
```suggestion
from the main file of each translation unit
are always displayed. Diagnostics from
non-system headers it includes are displayed
by default, but can be disabled with this option.
```
(Also applies in other places where the wording is repeated)

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