minor symfony#25879 Improve phpdoc to make it more explicit (mnapoli)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#25879).

Discussion
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Improve phpdoc to make it more explicit

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   |
| Fixed tickets |
| License       | MIT
| Doc PR        |

It confused me a lot that the method took an array and that the phpdoc said we should pass a string instead of an array. This small change should clear up the confusion.

Commits
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ecdbea2 Improve phpdoc to make it more explicit

Author: nicolas-grekas

src/Symfony/Component/Process/Process.php

@@ -1000,7 +1000,7 @@ public function getEnv()
     /**
      * Sets the environment variables.
      *
-     * An environment variable value should be a string.
+     * Each environment variable value should be a string.
      * If it is an array, the variable is ignored.
      *
      * That happens in PHP when 'argv' is registered into