Clarify meaning of element implicitness in documentation

**Describe the Bug**
The `isImplicit()` method sometimes returns a wrong value when dealing with implicit elements such as implicit constructors. (For the previous example the output is always `false`.)

**To Reproduce**

Input:
```java
package pkg;

public class TestClassImplicitConstructor { }
```

Processing with Spoon:
```java
public class Main {

public static void main(String[] args) {
SpoonAPI spoon = new Launcher();
spoon.addInputResource("src/main/java/pkg");
CtModel model = spoon.buildModel();

model.getAllTypes().forEach(t -> {
t.getAllExecutables().forEach(m -> {
if (m.isConstructor()) {
System.out.println(String.format(
"%s is implicit: %s",
m,
m.isImplicit()));
}
});
});
}
}
```

Output (Console):
```java
pkg.TestClassImplicitConstructor() is implicit: false
```

**Operating system, JDK and Spoon version used**

* OS: Ubuntu 21.04
* JDK: OpenJDK 17.0.1
* Spoon version: 9.0.0