Describe shipped presets and looks in the manual and FR-DEV-6

The presets sheet now lists the shipped collection beside the
photographer's own, a copy under a shipped name overrides it, and
shipped and imported presets are looks that leave a photograph's own
corrections alone. The manual says so where it introduces the sheet,
and FR-DEV-6 states the rule. The sheet's screenshot (media/presets.png)
predates the sections and needs re-recording.

docs/manual/index.html

@@ -318,6 +318,13 @@ black-and-white stocks at its end.</p>
 <p>Every change is a step; <code>Undo</code> and the History panel walk them. <code>Snapshot</code>
 keeps the current state under a name. <code>Presets…</code> saves the settings to
 apply elsewhere, and imports <code>.xmp</code> from other applications.</p>
+<p>The sheet lists your own presets first, then the ones DarkRoom ships —
+Essentials, and colour, cinema and black-and-white film, one measured stock
+each. A shipped preset is a look: it changes what it names and leaves the
+photograph's own corrections alone, as an imported Lightroom preset does.
+Saving under a shipped preset's name makes your version the one that name
+applies, marked <em>changed</em>; <code>Revert</code> brings the shipped one back, and renaming
+yours makes it one of your own.</p>
 <figure><img loading="lazy" src="media/presets.png" alt="The presets sheet"><figcaption>The presets sheet</figcaption></figure>
 <h3 id="copying-settings">Copying settings</h3>
 <p><code>Copy</code> in the top bar, or Ctrl+C, takes this photograph's settings; <code>Paste</code>,