Added some links

Author: javiereguiluz

http_cache/validation.rst

@@ -38,8 +38,8 @@ to implement the validation model: ``ETag`` and ``Last-Modified``.
 Validation with the ``ETag`` Header
 -----------------------------------
 
-The ``ETag`` ("entity-tag") header is an optional HTTP header whose value is an
-arbitrary string that uniquely identifies one representation of the target
+The `HTTP ETag`_ ("entity-tag") header is an optional HTTP header whose value is
+an arbitrary string that uniquely identifies one representation of the target
 resource. It's entirely generated and set by your application so that you can
 tell, for example, if the ``/about`` resource that's stored by the cache is
 up-to-date with what your application would return.
@@ -78,10 +78,10 @@ to 304.
 
     When using ``mod_deflate`` or ``mod_brotli`` in Apache 2.4, the original
     ``ETag`` value is modified (e.g. if ``ETag`` was ``foo``, Apache turns it
-    into ``foo-gzip`` or ``foo-brotli``), which breaks the ETag-based validation.
+    into ``foo-gzip`` or ``foo-br``), which breaks the ETag-based validation.
 
     Starting from Apache 2.5, you can control this behavior with the
-    ``DeflateAlterETag`` and ``BrotliAlterETag`` directives. Alternatively, you
+    `DeflateAlterETag`_ and `BrotliAlterETag`_ directives. Alternatively, you
     can use the following Apache configuration to keep both the original ETag
     and the modified one when compressing responses:
 
@@ -236,3 +236,6 @@ headers that must not be present for ``304`` responses (see
 :method:`Symfony\\Component\\HttpFoundation\\Response::setNotModified`).
 
 .. _`expiration model`: https://tools.ietf.org/html/rfc2616#section-13.2
+.. _`HTTP ETag`: https://en.wikipedia.org/wiki/HTTP_ETag
+.. _`DeflateAlterETag`: https://httpd.apache.org/docs/trunk/mod/mod_deflate.html
+.. _`BrotliAlterETag`: https://httpd.apache.org/docs/2.4/mod/mod_brotli.html