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

[Joseph Huber via cfe-commits]

================
@@ -1073,6 +1073,29 @@ 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 (``-fno-builtins``),
+* unwind tables are disabled (``fno-asynchronous-unwind-tables -fno-unwind-tables``),
+* allows ``main`` to be used as a regular function, and
+* removes implicit system header search paths and link libraries.
----------------
jhuber6 wrote:

That's my bad, I confused it with `-nostdlib` because they usually go together.

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