Rename ABI file

Author: johnno1962

docs/ABI/RegisterAllocation.md renamed to docs/ABI/RegisterUsage.md

@@ -1,4 +1,4 @@
-## 64 bit Architecture Register Allocations
+## 64 bit Architecture Register Usage
 
 From Swift 5, the calling convention register allocation for 64 architectures is largely based on [existing](https://developer.apple.com/library/content/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html) [standards](https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/LowLevelABI/140-x86-64_Function_Calling_Conventions/x86_64.html) with the addition of the context and error return registers discussed in the ABI manifesto:
 

docs/ABIStabilityManifesto.md

@@ -337,7 +337,7 @@ Having the call context register be callee-saved is advantageous. It keeps the r
 
 Throwing functions communicate error values to their callers through the *error* register on some platforms. The error register holds a pointer to the error value if an error occurred, otherwise 0. The caller of a throwing function is expected to quickly check for 0 before continuing on with non-error code, otherwise branching to code to handle or propagate the error. Using a callee-saved register for the error register enables free conversion from non-throwing to throwing functions, which is required to honor the subtyping relationship.
 
-The specific registers used in these roles is documented in [this document](ABI/RegisterAllocation.md).
+The specific registers used in these roles is documented in [this document](ABI/RegisterUsage.md).
 
 ### <a name="function-signature-lowering"></a>Function Signature Lowering