Add note on using S3's url option

Fix typo.

Progress on #2071.

[closes #2282]

[ci skip]

lib/paperclip.rb

@@ -120,7 +120,7 @@ module ClassMethods
     # called on it, the attachment will *not* be deleted until +save+ is called. See the
     # Paperclip::Attachment documentation for more specifics. There are a number of options
     # you can set to change the behavior of a Paperclip attachment:
-    # * +url+: The full URL of where the attachment is publically accessible. This can just
+    # * +url+: The full URL of where the attachment is publicly accessible. This can just
     #   as easily point to a directory served directly through Apache as it can to an action
     #   that can control permissions. You can specify the full domain and path, but usually
     #   just an absolute path is sufficient. The leading slash *must* be included manually for
@@ -129,6 +129,9 @@ module ClassMethods
     #   Paperclip::Attachment#interpolate for more information on variable interpolaton.
     #     :url => "/:class/:attachment/:id/:style_:filename"
     #     :url => "http://some.other.host/stuff/:class/:id_:extension"
+    #   Note: When using the +s3+ storage option, the +url+ option expects
+    #   particular values. See the Paperclip::Storage::S3#url documentation for
+    #   specifics.
     # * +default_url+: The URL that will be returned if there is no attachment assigned.
     #   This field is interpolated just as the url is. The default value is
     #   "/:attachment/:style/missing.png"