Update links from within the inline comments
Make web app manifest file comments more clear

Update inline comments regarding the web application manifest file to
point out that it is recommended (not mandatory) that this file should
be serve with the `application/manifest+json` media type.

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From http://w3c.github.io/manifest/:

  " If the protocol over which the manifest is transferred supports
    the [MIME-TYPES] specification (e.g. HTTP), it is RECOMMENDED that
    the manifest be labeled with the media type for a manifest. "

Thanks @marcoscaceres for pointing that out!

Ref: https://github.com/h5bp/server-configs-apache/issues/29#issuecomment-45154636