subnet configuration is confusing and undocumented

I feel we should have more documentation (ideally exposed in the web UI, perhaps via mouse over) to help users with subnet configuration. For me, at least, it involved performing hours of black box experimenting to try and understand it before I could configure it the way I wanted. I feel with a small amount of additional text, I could have configured MAAS correctly for my use case the first time. Some proposed text is included below but, as it is based on blackbox experimentation, maybe somewhat inaccurate.

= Reserved Ranges =
The "Reserved" section in subnets doesn't really tell me what I'm being asked to define. Reserved from what for what? At first I assumed these were ranges MAAS should not use - but why would MAAS then need to differentiate between "static" and "dynamic"? I had to fail several times before this started making sense. I had DHCP that wouldn't turn on due to lack of dynamic ranges, I had nodes that would fail to enlist/commission due to lack of "dynamic range" space, and I had nodes that would deploy but fail to boot due to lack of "available" space.

Reserve Range
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Proposed text: Define a range of IPs that MAAS should never assign. These IPs can be statically assigned to systems on your VLAN that MAAS will not manage.

Reserve dynamic range:
======================
Proposed text: Define a range of IPs that MAAS (or an externally configured DHCP server) will assign to unknown systems that request DHCP configuration in this VLAN. IPs in this range will be assigned both to systems that are installed outside of MAAS, as well as by MAAS to manage MAAS nodes in non-deployed phases (e.g. commissioning and deploying).

"Auto assign pool"
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There is a 3rd pool of IPs that is only implicit here - those are the IPs that are not reserved. IMO, we should make the existence and purpose of this pool explicit. Arguably, the "Availability" section is just that - but it isn't clear what purpose they are "available" for. At first I just assumed they were "available" for me to place in a reservation range. What I discovered during experimentation is that these IPs are used for interfaces that are set to "Auto assign". (Until then I also had no idea what the difference was between "Auto assign" and "DHCP"). "Auto assign pool" seems like it would more clearly describe these IPs, and give me a familiar term I'd recognize on the node/interface section.

It would even be better, IMO, to explicitly show the IP ranges in the auto assign pool in the UI. That way you could see that when you add a reserved range, the "auto assigned" range drops those IPs. But, I can see that being noisy w/ fragmented reserved ranges. A pie chart showing "reserved static"/"reserved dynamic" and "auto assign pool" would be a visually appealing way to see this relationship.

= DHCP =
DHCP activation feels very buried to me. I defined a dynamic IP range for my subnet but, when I boot a node on that subnet (e.g. for enlistment), it fails to DHCP!. Why would I define a reserved dynamic range and *not* want DHCP? (I'm guessing that this is to provide an option for a non-MAAS DHCP server). It would be helpful to warn the user that they need to go to the VLAN page and turn on DHCP if they expect MAAS to serve their dynamic ranges (or otherwise, they need to make sure to setup their own DHCP server). Maybe even provide a clickable link to the VLAN page so they don't have to dig for it.