document custom http endpoints for nodes

[mblackstock]

In creating nodes https://nodered.org/docs/creating-nodes/ probably based on stack overflow article
https://stackoverflow.com/questions/41567175/send-data-on-configuration/41567832#41567832

[MartinMuzatko]

having a full documentation here would already help: https://nodered.org/docs/api/runtime/api#httpAdmin