I fear that the section "invoking descriptor" was quite hard to grasp on first
reading. I try to improve this with the following changes.

The notion of lookup chain was used without ever being defined anywhere in the
documentation. I add a short definition.

> may override the default behavior and invoke the descriptor method instead

was changed. It seemed to indicate that descriptor replace the behavior
described previously, of checking for the value associated to `x` in `a` in the
lookup chain. To the best of my understanding, the lookup chain is still used,
since we do not even know whether `x` is a descriptor or not until the lookup
chain returned a value. It would be more exact to indicate that a descriptor
override the action of returning, assigning or deleting.

> The starting point for descriptor invocation is a binding, ``a.x``. How the
> arguments are assembled depends on ``a``:

While python#29909 was a net improvement, I fear it was not sufficient. I had three
issues while reading this section for a first time.
* The Direct Call actually do not use the binding `a.x` (that's the very point
of the direct call), so the Direct Call should not have been in this
enumeration. In fact, what is even worse is that in the Direct Call, `x` is a
variable while in all other case, `x` is an attribute name.
* Furthermore, once I was reading the Super Binding, it was confusing that what
was called `a` in the introduction of the list is now called `super(A, a)`. At
this point, I believe it is more clear to just state that there are four ways to
invoke a descriptor and to list them. Each case introducing the variables it
requires.pp
* Super Binding uses the name "dotted lookup" while the remaining of the
documentation uses "attribute access". This lead me to assume that there is a
reason why a different name was used, that it was referencing a different
concept. I believe it is best to be consistent with naming of a concept.
* Super Binding "for a base class ``B`` following ``A``" was slightly confusing,
as it seems to indicates that there may exists multiple such base class `B`. I
believe that "the base class ``B``" is clearer.

For your information "dotted lookup" is also used in the howto section; I'd
suggest to change it there too for the sake of consistency. But since the usage
is consistent throughout the howto's descriptor file, this is at least less
confusing than this usage.