Name the re-included env templates in the .gitignore comment

The comment said a committed template is re-included by a negation, but
the file re-includes only `example.env` and `sample.env`. It now names
those two and tells a repository to add its own negation after these
lines for any other template, for example `!.env.example`, as the
`.dockerignore` comment does.

Model: opus-5-5

.gitignore

@@ -25,8 +25,9 @@ node_modules/
 # ranges rather than a lowercase form that misses `Server.Key`.
 
 # Environment files. `*.env` covers bare `.env` and the `prod.env`
-# convention. A file of this shape committed on purpose, such as a
-# template holding no real values, is re-included by a negation.
+# convention. Only the templates `example.env` and `sample.env` are
+# re-included below. A repository that commits any other template adds
+# its own negation after these lines, for example `!.env.example`.
 *.[eE][nN][vV]
 .[eE][nN][vV].*
 .[eE][nN][vV][rR][cC]