Clarify use of bind operator in "prefix position" (#5917)

The REPL [shows](https://babeljs.io/repl/#?babili=false&evaluate=false&lineWrap=true&presets=es2015%2Creact%2Cstage-0&targets=&browsers=&builtIns=false&debug=false&code_lz=PYIwVgXBBmCuB2BjA3AKAPToAQEsDOWApgI6w4BuAhgDaHwAuW9wEqcSAdCDvACYAUoMAEo0qKEI7sUGbPiKkKNOo2atJ0rjwFDRqVEKjT-Vansy4CJMqZVMWbBIg6Ia1QeAA0WU3vEQNJxMaczkrRVsGe1ZNV2p3IW9fZCA)
that when the bind operator prefixes `obj.func` (as opposed to being used
between `obj` and `func`), rather than binding a free function `func` to `obj`,
it binds `obj.func` to `obj`.

[skip ci]

packages/babel-plugin-transform-function-bind/README.md

@@ -9,13 +9,17 @@ obj::func
 // is equivalent to:
 func.bind(obj)
 
+::obj.func
+// is equivalent to:
+obj.func.bind(obj)
+
 obj::func(val)
 // is equivalent to:
 func.call(obj, val)
 
 ::obj.func(val)
 // is equivalent to:
-func.call(obj, val)
+obj.func.call(obj, val)
 ```