minor #11999 [Mailer] Added a note about using absolute URLs in mailer (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes #11999).

Discussion
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[Mailer] Added a note about using absolute URLs in mailer

Fixes #11995.

Commits
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5438aab [Mailer] Added a note about using absolute URLs in mailer

Author: OskarStark

mailer.rst

@@ -200,7 +200,8 @@ Use the ``attachFromPath()`` method to attach files that exist on your file syst
         ->attachFromPath('/path/to/documents/privacy.pdf', 'Privacy Policy')
         // optionally you can provide an explicit MIME type (otherwise it's guessed)
         ->attachFromPath('/path/to/documents/contract.doc', 'Contract', 'application/msword')
-        // you can also use an absolute url
+        // you can also use an absolute URL if your PHP config allows getting URLs using fopen()
+        // (this is not recommended because your application may or may not work depending on PHP config)
         ->attachFromPath('http://example.com/path/to/documents/contract.doc', 'Contract', 'application/msword')
     ;