[doc] Editing for better style (#1736)

* Also removed references to Persona authentication.
* Changed code comment about Persona because it's for HTTP auth
  in general. See https://github.com/FreshRSS/FreshRSS/commit/3d876091e1268e3ccd5036449a4deb5134936206
  and https://github.com/FreshRSS/FreshRSS/issues/358#issuecomment-31931484
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