minor symfony#8753 [FIX] Realpath improvement in PHP 7.2 (Guikingone, weaverryan)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#8753).

Discussion
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[FIX] Realpath improvement in PHP 7.2

As PHP 7.2 provide a default value of 4096k (as seen https://github.com/php/php-src/blob/php-7.2.0RC6/UPGRADING#L357), the line about this improvement can be deleted.

Commits
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f10c7b3 tweaking language
a519d8e fix(syntax): added a space before has
17a0b8a fix(tip): Add the tip block for realpth_cache_size
e177ca9 [FIX](Realpath)

Author: weaverryan

performance.rst

@@ -79,14 +79,12 @@ real and absolute file system paths. This increases the performance for
 applications like Symfony that open many PHP files, especially on Windows
 systems.
 
-By default, PHP sets a ``realpath_cache_size`` of ``16K`` which is too low for
-Symfony. Consider updating this value at least to ``4096K``. In addition, cached
-paths are only stored for ``120`` seconds by default. Consider updating this
-value too using the ``realpath_cache_ttl`` option:
+Consider increasing the ``realpath_cache_size`` and ``realpath_cache_ttl``:
 
 .. code-block:: ini
 
     ; php.ini
+    ; 4096k is the default value in PHP 7.2
     realpath_cache_size=4096K
     realpath_cache_ttl=600