minor #4655 Improve docs - README (mvorisek)

This PR was submitted for the master branch but it was merged into the 2.15 branch instead (closes #4655).

Discussion
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Improve docs - README

Based on https://github.com/symfony/symfony/blob/1b4ab8108542daddd6ca96caf38b464ff3cc459a/src/Symfony/Component/Finder/Finder.php#L317

Commits
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6ba5cbd Improve docs - README

README.rst

@@ -1934,6 +1934,7 @@ The example below will add two rules to the default list of PSR2 set rules:
     ;
 
 **NOTE**: ``exclude`` will work only for directories, so if you need to exclude file, try ``notPath``.
+Both ``exclude`` and ``notPath`` methods accept only relative paths to the ones defined with the ``in`` method.
 
 See `Symfony\\Finder <https://symfony.com/doc/current/components/finder.html>`_
 online documentation for other `Finder` methods.

src/Console/Command/HelpCommand.php

@@ -179,6 +179,7 @@ public static function getHelpCopy()
     ?>
 
 **NOTE**: ``exclude`` will work only for directories, so if you need to exclude file, try ``notPath``.
+Both ``exclude`` and ``notPath`` methods accept only relative paths to the ones defined with the ``in`` method.
 
 See `Symfony\Finder` (<url>https://symfony.com/doc/current/components/finder.html</url>)
 online documentation for other `Finder` methods.