Document client special registers

An example set of SPRs looks like:

```properties
V_CLIENT_APPLNAME=VSCode
V_CLIENT_ACCTNG=location: file:/path/to/mapepire.jar
V_CLIENT_PROGRAMID=VSCode connector | Version 2.1.7
V_CLIENT_WRKSTNNAME=67.210.225.163
V_CLIENT_USERID=THEPREZ
```
