x/cabi: improve Go package documentation

Author: ydnar

x/cabi/README.md

@@ -0,0 +1,19 @@
+// Package cabi contains a single WebAssembly function exported as cabi_realloc.
+//
+// To use, import this package with _:
+//
+//	import _ "go.bytecodealliance.org/x/cabi"
+//
+// Function realloc is a WebAssembly [core function] that is validated to have the following core function type:
+//
+//	(func (param $originalPtr i32)
+//	      (param $originalSize i32)
+//	      (param $alignment i32)
+//	      (param $newSize i32)
+//	      (result i32))
+//
+// The [Canonical ABI] will use realloc both to allocate (passing 0 for the first two parameters) and reallocate. If the Canonical ABI needs realloc, validation requires this option to be present (there is no default).
+//
+// [core function]: https://www.w3.org/TR/wasm-core-2/syntax/modules.html#functions
+// [Canonical ABI]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/CanonicalABI.md
+package cabi