stray references to IPAM

[rade]

We have exactly three references to 'IPAM' in our docs:

proxy.md:113:If you do not want IPAM to be used by default, the proxy needs to be
proxy.md:189:this example with an address allocated by IPAM. If you wish you can
weavedns.md:75:allocated automatically by IPAM from the default subnet; you can

Nowhere do we explain what IPAM stands for. We tend to spell it out instead, as "automatic IP address management/allocator", or just "IP allocator".

The only other place where we use ipam is in the --no-default-ipam proxy option. We may want to change that.