[clang-tools-extra] [doc] Add documentation for clang-change-namespace (PR #148277)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,155 @@
+======================
+Clang-Change-Namespace
+======================
+
+.. contents::
+
+.. toctree::
+  :maxdepth: 1
+
+:program:`clang-change-namespace` can be used to change the surrounding
+namespaces of class/function definitions.
+
+Classes/functions in the moved namespace will have new namespaces while
+references to symbols (e.g. types, functions) which are not defined in the
+changed namespace will be correctly qualified by prepending namespace specifiers
+before them. This will try to add shortest namespace specifiers possible.
+
+When a symbol reference needs to be fully-qualified, this adds a `::` prefix to
+the namespace specifiers unless the new namespace is the global namespace. For
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AaronBallman wrote:

> I'd say it is not up to the tool to ensure compilability in that sense. Of course, if you agree to this, then this MUST be documented.

I think that's worth documenting.

https://github.com/llvm/llvm-project/pull/148277