add references to custom ret-sync plugins

[saidelike]

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

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

Other (specify)

Disassembler version

N/A

Disassembler log output

Operating system versiom

Windows