Docs talk about removed "Leaf types"

[Seelengrab]

isleaftype was removed in 2018 in #25496, but unfortunately the FFI docs still refer to the concept:

julia/doc/src/manual/calling-c-and-fortran-code.md

Line 281 in 59f08df

| `mutable struct` | `BitSet` | "Leaf Type" :: A group of related data that includes a type-tag, is managed by the Julia GC, and is defined by object-identity. The type parameters of a leaf type must be fully defined (no `TypeVars` are allowed) in order for the instance to be constructed. |

julia/doc/src/manual/calling-c-and-fortran-code.md

Line 288 in 59f08df

| `struct ...; end` | `nothing` | "Singleton" :: a Leaf Type or Struct with no fields. |

julia/doc/src/manual/calling-c-and-fortran-code.md

Line 629 in 59f08df

* `T`, where `T` is a Julia leaf type

julia/doc/src/manual/calling-c-and-fortran-code.md

Line 682 in 59f08df

* `T`, where `T` is a Julia Leaf Type

As do the performance tips:

julia/doc/src/manual/performance-tips.md

Line 808 in 59f08df

The following examples may help you interpret expressions marked as containing non-leaf types:

julia/doc/src/manual/performance-tips.md

Line 824 in 59f08df

* Interpretation: getting a field that is of non-leaf type. In this case, the type of `x`, say `ArrayContainer`, had a

The performance docs are particularly unfortunate, because they don't introduce the term and don't point to another place that has a correct definition. Further, since "leaf type-ness" is not queryable anymore, the ccalling docs end up being a bit confusing in terms of what a corresponding type would even be.

There's this comment by @vtjnash clarifying the situation back when this was decided, so that'd be a good starting point for finding a wording that is both accurate & uses up-to-date terminology.

[vtjnash]

That looks like it should have been corrected to say isconcretetype at the time

[Seelengrab]

In both the FFI docs & performance tips?