minor #17437 [Finder] Improve the phpdoc of SplFileInfo methods (mnapoli)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #17437).

Discussion
----------

[Finder] Improve the phpdoc of SplFileInfo methods

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I **always** get confused when I need to use the finder (getFilename, getPath, getPathname, getRelativePath, getRelativePathname, …). This is only addressing the last two methods but hopefully it will help.

Commits
-------

1b170d7 Improve the phpdoc of SplFileInfo methods

src/Symfony/Component/Finder/SplFileInfo.php

@@ -38,6 +38,8 @@ class SplFileInfo extends \SplFileInfo
     /**
      * Returns the relative path.
      *
+     * This path does not contain the file name.
+     *
      * @return string the relative path
      */
     public function getRelativePath()
@@ -48,6 +50,8 @@ class SplFileInfo extends \SplFileInfo
     /**
      * Returns the relative path name.
      *
+     * This path contains the file name.
+     *
      * @return string the relative path name
      */
     public function getRelativePathname()