[clang] [Docs] Document freestanding requirements (PR #132232)

[Vlad Serebrennikov via cfe-commits]

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@@ -1073,6 +1073,35 @@ inputs. Here is some example of ``$``-prefixed options:
 Language and Target-Independent Features
 ========================================
 
+Freestanding Builds
+-------------------
+Passing the ``-ffreestanding`` flag causes Clang to build for a freestanding
+(rather than a hosted) environment. The flag has the following effects:
+
+* the ``__STDC_HOSTED__`` predefined macro will expand to ``0``,
+* builtin functions are disabled by default (``-fno-builtins``),
+* unwind tables are disabled by default 
+  (``fno-asynchronous-unwind-tables -fno-unwind-tables``), and
+* allows ``main`` to be used as a regular symbol.
----------------
Endilll wrote:

It might be worth expanding on what being a regular symbol entails. I guess it's the ability to take address of it and the ability to call it, but it's not exactly clear to me, either.

https://github.com/llvm/llvm-project/pull/132232