[Docs] Clarify `html` encoder in highlighting.asciidoc (#27766)

The previous description was a bit confusing because the pre/post tags used for highlighting are not escaped, the rest of the content is.

docs/reference/search/request/highlighting.asciidoc

@@ -142,8 +142,9 @@ You can specify the locale to use with `boundary_scanner_locale`.
 boundary_scanner_locale:: Controls which locale is used to search for sentence
 and word boundaries.
 
-encoder:: Indicates if the highlighted text should be HTML encoded:
-`default` (no encoding) or `html` (escapes HTML highlighting tags).
+encoder:: Indicates if the snippet should be HTML encoded:
+`default` (no encoding) or `html` (HTML-escape the snippet text and then
+insert the highlighting tags)
 
 fields:: Specifies the fields to retrieve highlights for. You can use wildcards
 to specify fields. For example, you could specify `comment_*` to