Fix hx-swap-oob-only responses wiping main target + document behavior

[scriptogre]

__processMainSwap doesn't behave consistently for responses that only contain <hx-patial>s vs only hx-swap-oob:

<!-- This response would wipe `hx-target` if `hx-swap="innerHTML"` -->
<div id="notifications" hx-swap-oob="true">3 new messages</div>

<!-- But this one wouldn't -->
<hx-partial hx-target="#notifications">3 new messages</hx-partial>

Fix: Make behaviour consistent (skip main target in both scenarios), and make the code more obvious about it.

Also documents "responses without main content" behavior in the hx-swap-oob reference page, docs, and a new <hx-partial> reference page.