jails: Fix incorrect VNET instructions

[dch]

Summary

The /etc/rc step must be moved from the first position in the jail
startup, to the last, or daemons will start before the epair networking
is established.

[ziaee]

Please use a commit message like handbook/jails: Fix VNET instructions.

But wait,

1. Can you usually use += even if its empty? Should we fix the code instead? Adding Jails to investigate.

2. Should we document this somewhere in an explanation instead of a remedial way, e.g. in command hooks section starting on line 508?

[jamie]

In D60992#1398733, @ziaee wrote:

But wait,

1. Can you usually use += even if its empty? Should we fix the code instead? Adding Jails to investigate.

Sure, += for an empty variable is reasonable: it's appending to a zero-length array.

[dch]

Expand to hooks section, explain issue better, and its consequences.

[dch]

with a jails hat on, there is typically a default exec.start set, by a template or an include. This is mentioned previously in the document. I've expanded both the hooks section and also the comment itself. Does it read better now?

[ziaee]

Yeah I like it