add references to custom ret-sync plugins

### What happened?

ret-sync custom plugins exist but are not listed in the docs eg https://doc.esreven.eshard.com/enterprise/2023.04/Axion/Axion-Plugin-Ret-Sync.html

### What debugger(s) are you seeing the problem on?

Other (specify)

### Debugger version

Reven

### Debugger log output

```shell

```

### What disassembler(s) are you seeing the problem on?

Other (specify)

### Disassembler version

N/A

### Disassembler log output

```shell

```

### Operating system versiom

Windows