Allow joining complex searches using "AND" and document search better

[adelavega]

"AND" is not behaving as expected when joining complex "or" searches.

A user wrote to us via appzi as follows:

"I tested the following sets of keywords and noticed some surprising results:
Set 1: "autism"
This search yielded 608 results on Neurosynth Compose
Set 2: ❨"autism" OR "ASD" OR "autistic"❩ AND ❨"decision" OR "option"❩
This search yielded 867 results on Neurosynth Compose
For comparison, I ran similar searches on the PubMed search engine:
A search for "autism" yields around 90,000 results
Adding the additional keywords with AND constraints, i.e. ❨"autism" OR "ASD" OR "autistic"❩ AND ❨"decision" OR "option"❩, reduces the results to around 2,000
The PubMed behavior aligns with my expectations, as adding more constraints with the AND operator should reduce the number of matching articles. However, I was surprised to see that on Neurosynth Compose, adding those additional keywords actually increased the number of results from 608 to 867."

Looking into this I realized that "and" after a set of ors in parantheses does not work as expected:

"However, unfortunately, it looks like we don't currently support chaining operators together.For example: (autism or ASD or autistic) and sdlfkjldsf results in the same number of hits, which tells me the last term is being treated as an additional "or"."

If we can implement PubMed like search we should, otherwise let's document well how our search works.
Allowing users to join two complex searches using an AND (i..e get the set that meets both criteria) would be very useful going forward.