Searching feeds and (proposed) searching content are close but different

Hard to find a title on this one.

The spec mentions a POST for `action=search&query=aaronparecki.com` as a way to search feeds to subscribe on. Together is using this feature to find feeds to subscribe to.

The spec also mentions a POST for `action=search&channel=x-uid-x&query=words`, as a way to search within a channel for certain posts. They differ by the addition of `channel`, which is a very small difference, while they are very different endpoints.

I would argue that the first one is more closely related to `action=preview&url=xx` and that it could be better represented as `action=preview&query=aaronparecki.com`.

In a non-logged chat @aaronpk noted that the search actually is more a search than a preview, because in the example (below) 'percolator.today' is only found because 'aaronparecki.com' is in the response. So it's actually a search.

> ```
> POST /microsub
> Content-type: application/x-www-form-urlencoded
>
> action=search&query=aaronparecki.com
> ```
> Response
> ```
> HTTP/1.1 200 Ok
> Content-type: application/json
>
>
> {
> "results": [
> {
> "type": "feed",
> "url": "https://aaronparecki.com/",
> "name": "Aaron Parecki",
> "photo": "https://aaronparecki.com/images/profile.jpg",
> "description": "Aaron Parecki's home page"
> },
> {
> "type": "feed",
> "url": "https://percolator.today/",
> "name": "Percolator",
> "photo": "https://percolator.today/images/cover.jpg",
> "description": "A Microcast by Aaron Parecki",
> "author": {
> "name": "Aaron Parecki",
> "url": "https://aaronparecki.com/",
> "photo": "https://aaronparecki.com/images/profile.jpg"
> }
> },
> { ... }
> ]
> }
> ```

Start by reading the Microsub specification sections covering POST /microsub with action=search and action=preview, then compare the feed-search and channel-search examples in this issue. Done means the distinction and intended request shapes are resolved and documented without ambiguity.

Written by the indexing model from the issue text.