[llvm] [docs][NFC] Add usage instructions for FatLTO (PR #75853)

[Fangrui Song via llvm-commits]

================
@@ -76,3 +76,40 @@ Supported File Formats
 The current implementation only supports ELF files. At time of writing, it is
 unclear if it will be useful to support other object file formats like ``COFF``
 or ``Mach-O``.
+
+Usage
+===========
----------------
MaskRay wrote:

Shorten `===========`?

https://github.com/llvm/llvm-project/pull/75853