OutputPage: documentation for (semi-)deprecated methods

OutputPage: documentation for (semi-)deprecated methods

Deprecated: addExtensionStyle, getExtStyle
These had been deprecated unofficially for a while now. ResourceLoader
is preferred.

Internal/Unpreferred: addHeadItem, addScript, addScriptFile,
addInlineScript, addStyle, addInlineStyle.
All should be used as littel as possible, in favor of ResourceLoader
and a few other higher level accessors. They are not truly deprecated,
but should not really be used unless you have a really good reason.

Also added a few missing wfDeprecated calls to methods marked as
deprecated in the phpdoc.

Bug: T89955
Change-Id: I472d906b6de16257fa34a2f9bfe9e4eb284d02b4