[Docs] Document --lto-whole-program-visibility

Summary:
Documents interaction of linker option added in D71913 with LTO
visibility.

Reviewers: pcc

Subscribers: inglorion, hiraditya, steven_wu, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75655

clang/docs/LTOVisibility.rst

@@ -35,6 +35,16 @@ other classes receive hidden LTO visibility. Classes with internal linkage
 (e.g. classes declared in unnamed namespaces) also receive hidden LTO
 visibility.
 
+During the LTO link, all classes with public LTO visibility will be refined
+to hidden LTO visibility when the ``--lto-whole-program-visibility`` lld linker
+option is applied (``-plugin-opt=whole-program-visibility`` for gold). This flag
+can be used to defer specifying whether classes have hidden LTO visibility until
+link time, to allow bitcode objects to be shared by different LTO links.
+Due to an implementation limitation, symbols associated with classes with hidden
+LTO visibility may still be exported from the binary when using this flag. It is
+unsafe to refer to these symbols, and their visibility may be relaxed to hidden
+in a future compiler release.
+
 A class defined in a translation unit built without LTO receives public
 LTO visibility regardless of its object file visibility, linkage or other
 attributes.