[llvm] [Offload] Improve liboffload documentation (PR #142403)

[Kenneth Benzie via llvm-commits]

================
@@ -1,24 +1,48 @@
 # Offload API definitions
 
-**Note**: This is a work-in-progress. It is loosely based on equivalent
-tooling in Unified Runtime.
-
 The Tablegen files in this directory are used to define the Offload API. They
 are used with the `offload-tblgen` tool to generate API headers, print headers,
 and other implementation details.
 
 The root file is `OffloadAPI.td` - additional `.td` files can be included in
 this file to add them to the API.
 
+## Modifying the API
----------------
kbenzie wrote:

This doesn't have a new line before the paragraph

https://github.com/llvm/llvm-project/pull/142403