[llvm] [llvm] add documentation for public interface annotations (LLVM_ABI, etc) (PR #134710)

[Saleem Abdulrasool via llvm-commits]

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+LLVM Interface Export Annotations
+=================================
+Symbols that are part of LLVM's public interface must be explicitly annotated
+to support shared library builds with hidden default symbol visibility. This
+document provides background and guidelines for annotating the codebase.
+
+LLVM Shared Library
+-------------------
+LLVM builds as a static library by default, but it can also be built as a shared
+library with the following configuration:
+
+::
+
+   LLVM_BUILD_LLVM_DYLIB=On
+   LLVM_LINK_LLVM_DYLIB=On
+
+There are three shared library executable formats we're interested in: PE
+Dynamic Link Library (.DLL) on Windows, Mach-O Shared Object (.dylib) on Apple
----------------
compnerd wrote:

```suggestion
Dynamic Link Library (.dll) on Windows, Mach-O Shared Object (.dylib) on Apple
```

https://github.com/llvm/llvm-project/pull/134710