Do not treat line-separated comments as docblocks

This is already handled by the `is-docblock` predicate, but was also needed
here.  In particular, if we have a situation like this:

```xml
<!--
  No a docblock.
-->

<function name="foo">
  <!-- ... -->
</function>
```

we do not want that comment to be considered the docblock of `foo`, since it
does not appear immediately adjacent to it.

src/texinfo.xsl

@@ -192,7 +192,8 @@
   <param name="context" as="node()" />
 
   <sequence select="( ( $context/preceding-sibling::node()[
-                          not( . instance of text() ) ] )
+                          not( . instance of text()
+                               and . = $xt:nl ) ] )
                         [ last() ] )
                       [ . instance of comment() ]" />
 </function>