The Help:Searching documentation for boolean search operators (AND, OR) describes how things might be expected to work, but it is far from the way things actually work.
In particular, parens are completely ignored (so (blue OR red) AND green and blue OR (red AND green) in fact give identical results) and logical operators are mapped in an unexpected way to Lucene MUST and SHOULD queries (so blue OR red AND green doesn't do either of the things you would expect it to do).
The current plan is to write a short bit of documentation for the Help page saying that parens are ignored, boolean AND and OR do not behave in the expected way and should probably not be used for the time being, and that the plan it to implement them properly in the future—along with a link to a longer page explaining how things actually work, including how queries are parsed and converted to MUST and SHOULD queries, explaining what those actually do, and expanding on our future plans to support proper boolean queries and explicit/intentional SHOULD queries.
mw:Help:CirrusSearch should also be reviewed and updated as necessary.
Once we properly support boolean queries and parens, we can change the Help:Searching docs back to what they are now, more or less, and put a big disclaimer on the more detailed page that it works like it is supposed to.