The include_retweets parameter is a mostly historical artifact in API v1.1. It was originally introduced in various methods of API v1, including the classic search.twitter.com API, as retweets weren’t originally part of the API and you had to “opt-in” to receive their special structure.
In API v1.1 retweets are included by default. Not all methods support the ability to disinclude them with include_retweets=false.
The Search API actually is an API that doesn’t support this parameter, which is why it’s not part of its documentation. Retweets (tweets with retweeted_status objects embedded inside them) will be returned for queries regardless of you specifying the parameter. If you don’t want to include retweets in your search results, you’ll need to post-filter them from the results you receive.