minor #7014 Add a note about Filesystem:mkdir behavior (mickaelandrieu)

This PR was merged into the 2.7 branch.

Discussion
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Add a note about Filesystem:mkdir behavior

This contribution is valid from 2.7 to 3.2+

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8d08a26 Add a note about Filesystem:mkdir behavior

Author: wouterj

components/filesystem.rst

@@ -64,6 +64,10 @@ On POSIX filesystems, directories are created with a default mode value
     You can pass an array or any :phpclass:`Traversable` object as the first
     argument.
 
+.. note::
+
+    This function ignores already existing directories.
+
 exists
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