SOLR-18030: Improve MLT docs

[epugh]

https://issues.apache.org/jira/browse/SOLR-18030

Is there a good reason why the MLT request handler isn't enabled by default? It's something we call out in the docs.

Also, I can't get the `{!mlt_content}lorem ipsum} to work. I tried

http://localhost:8983/solr/gettingstarted/mlt?mlt.fl=author&mlt.interestingTerms=details&mlt.match.include=true&mlt.mindf=0&mlt.mintf=0&q={!mlt_content q='george'}

and had no luck.

[mkhludnev]

Hi Eric!
My feeling was that MLT handler is a little bit of legacy and probably limited and superseded by MLT query parser.
In the former request they are used both handler and parser. I'd rather thy to send MLT query via parser into the regular handler.

[mkhludnev]

btw, how much the distributed support one can evidence here https://github.com/apache/solr/blob/main/solr/core/src/java/org/apache/solr/handler/MoreLikeThisHandler.java? It's interesting if it can be used for subordinate searches via shards.qt or so one.

[epugh]

I tested http://localhost:8983/solr/gettingstarted/select?q={!mlt qf=author mintf=1 mindf=1}0553573403 and it worked...

However, I tried making mlt.interestingTerms=list work with the !mlt and no joy. I also can't get the !mlt_content to work. Could you provide some more examples of queries? I'd be happy to add them to the docs.. I'm just somewhat struggling to understand what works, and potentially, are things I am bumping into actual bugs, or just me not using the tools properly!

[mkhludnev]

Did you pass qf via local param at {!mlt_content} ?
Note the ticket is https://issues.apache.org/jira/browse/SOLR-16420
Example queries are https://github.com/apache/solr/blob/main/solr/core/src/test/org/apache/solr/search/mlt/CloudMLTQContentParserTest.java
I briefly looked at the code with regards to mlt.interestingTerms I don't think qparsers deals somehow with it.

[epugh]

The docs are better, though not perfect. I want to get this in as I don't plan on doing anything else with it.