Update docs for AllowUserCssPrefs to not mention performance

Update docs for AllowUserCssPrefs to not mention performance

* Before 2011, this was implemented as an inline stylesheet in OutputPage.
* In 2011, this was re-implemented through the 'user.cssprefs' module
  (class: ResourceLoaderUserCSSPrefsModule) embedded in OutputPage.
* Between 2013 and 2016, all user-css preferences have either been
  removed or migrarated to not need this module, eventually resulting
  in the removal of that module in 2c688cfb11ea6c.

More details on T127328, but suffice it to say, it has no significant
performance concerns currently.

Change-Id: Ia89c4f91ecb67e3116989f2ea97c7ca69275a2fb