Delete the entire "x64 Unwinding of an ARM64EC function" section.

All the information within is incorrect. The idea of using x64 unwind opcodes for EC code was abandoned very early in the design phase of EC. EC code uses Arm64 unwind opcodes.

Author: pmsjt

docs/build/arm64ec-windows-abi-conventions.md

@@ -166,12 +166,6 @@ When the helper then returns to the thunk, the thunk:
 1. Pops the ARM64EC `lr` register
 1. Executes an ARM64 `ret lr` instruction.
 
-## x64 Unwinding of an ARM64EC function
-
-### Recovering the return address
-
-At the beginning of an x64 function, the stack pointer points to the return address that was pushed onto the stack by the caller's `call` instruction. In contrast, at the beginning of an ARM64EC function, the return address is in the `lr` register, set by the caller's `bl` instruction. Therefore, when unwinding the deepest frame of a stack, if the frame corresponds to an ARM64EC function, the x64 unwinder must recreate the value of ARM64EC `lr` by reading the value stashed in the x87 registers when the exception occurred. In addition, the new x64 unwind code `UWOP_SAVE_RET` handles the prolog saving ARM64EC `lr` to the stack.
-
 ## ARM64EC function name decoration
 
 An ARM64EC function name has a secondary decoration applied after any language-specific decoration. For functions with C linkage (whether compiled as C or by using `extern "C"`), a `#` is prepended to the name. For C++ decorated functions, a `$$h` tag is inserted into the name.