Change Description and Recommended Behavior of Search Action

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https://indieweb.org/Microsub-spec#Search

As discussed at https://indieweb.org/2021/Pop-ups/Microsub, this would be a shift from the paradigm of following feeds to following people.

* Search parameter should be explained that it does not need to be a feed URL. It could be a name looked up in a contact list, for example, or any other discovery methodology.
* We could instead focus on entering h-card url the endpoint could figure out how to derive feeds from h-card in order to emphasize the following of people.
* The endpoint could be more opinionated and return the feed it thinks best represents the identity, as opposed to all possibly found feeds. There may need to be separate discussion outside the spec re how to best find this feed.

This also has implications on the client side, where currently, most clients are letting users choose from multiple options.

Spec should recommend clients display the author property of the feed over the URL.

Start with the Microsub-spec Search section and the linked 2021 Pop-ups/Microsub discussion. Resolve the proposed search and feed-selection behavior before updating the specification, including the client recommendation to display a feed's author rather than its URL; done means the revised behavior is documented consistently.

Written by the indexing model from the issue text.