docs: give the last step of the prevnext chain its way back

docs/Licenses.wikitext

@@ -23,3 +23,5 @@ __TOC__
 <!--T:3-->
 The standalone binary is compiled with [https://frankenphp.dev/ FrankenPHP], which also links several Caddy modules; see the FrankenPHP project for their licenses. The Docker image keeps MediaWiki's own <code>COPYING</code>, and each component's full license text is available from its project.
 </translate>
+
+{{prevnext|Writing an extension|}}

docs/Template:prevnext.wikitext

@@ -1,8 +1,6 @@
 <templatestyles src="prevnext/styles.css" /><div class="wikven-prevnext"><!--
-  -->{{#if:{{{2|}}}|
-    <div class="wikven-prevnext-prev">&larr;&nbsp;[[Special:MyLanguage/{{{1|}}}|{{prevnext/label|{{{1|}}}}}]]</div>
-  }}<!--
-  --><div class="wikven-prevnext-next">[[Special:MyLanguage/{{{2|{{{1|}}}}}}|{{prevnext/label|{{{2|{{{1|}}}}}}}}]]&nbsp;&rarr;</div><!--
+  -->{{#if:{{{1|}}}|<div class="wikven-prevnext-prev">&larr;&nbsp;[[Special:MyLanguage/{{{1}}}|{{prevnext/label|{{{1}}}}}]]</div>}}<!--
+  -->{{#if:{{{2|}}}|<div class="wikven-prevnext-next">[[Special:MyLanguage/{{{2}}}|{{prevnext/label|{{{2}}}}}]]&nbsp;&rarr;</div>}}<!--
 --></div><noinclude>
 {{prevnext/doc}}
 </noinclude>

docs/Template:prevnext/doc.wikitext

@@ -2,11 +2,14 @@ Bottom-of-page navigation between the steps of the documentation, as a row of on
 
 == Usage ==
 
- <nowiki>{{prevnext|next step}}
-{{prevnext|previous step|next step}}</nowiki>
-
-Each argument is a page name, not a link label: it is the link target, and the link text is the
-target's own title.
+ <nowiki>{{prevnext|previous step|next step}}
+{{prevnext||next step}}
+{{prevnext|previous step|}}</nowiki>
+
+The first argument is always the step before this page and the second always the step after it, so
+an end of the chain leaves its own side empty: the first page has nothing before it, the last has
+nothing after it. Both are page names rather than link labels — the name is the link target, and
+the link text is the target's own title.
 
 == Translated pages ==
 

docs/Why wikitext.wikitext

@@ -30,4 +30,4 @@ __TOC__
 That last cost, running MediaWiki, is the one {{SITENAME}} removes: it renders your wikitext through MediaWiki once, at build time, and ships plain static HTML. You get templates, parser functions and the rest with nothing to run in production. Reach for {{SITENAME}} when that trade is worth it, typically when your content is already wikitext, or when templates and transclusion would save real work.
 </translate>
 
-{{prevnext|Installation}}
+{{prevnext||Installation}}

docs/index.wikitext

@@ -54,5 +54,5 @@ These need a live wiki, so they are not part of the static export:
 What replaces some of them is written down: the reader's own display choices are on a [[<tvar name="1">Special:MyLanguage/Skins#Settings</tvar>|Settings page]] the build writes, and searching is [[<tvar name="2">Special:MyLanguage/Searching</tvar>|done in the browser]].
 </translate>
 
-{{prevnext|Why wikitext}}
+{{prevnext||Why wikitext}}
 {{DISPLAYTITLE:Wikven}}