Clarify documentation for TypeBoost & HtmlFilter processors

In Solr 7 document boosting aka index time boosting support has been dropped entirely.

This feature was used for the TypeBoost & HtmlFilter processors. In Search API Solr a similar feature has been implemented that gives the same result and uses lucene payloads.

When setting the boost to 0 for a bundle in the TypeBoost SearchApiProcessor the score for all documents of that bundle will be 0!

For example:

[details] => Array
                        (
                            [0] => Array
                                (
                                    [match] => 1
                                    [value] => 8.615736
                                    [description] => sum of:
                                    [details] => Array
                                        (
                                            [0] => Array(4)
                                        )

                                )

                            [1] => Array
                                (
                                    [match] => 1
                                    [value] => 0
                                    [description] => float(boost_document)=0.0
                                )

                        )

The boost factor for the entire document is 0. 8.615736*0=0

We should update the description in the UI to make this clear to the end user.

For HtmlFilter it now reads:
"Assign a boost of 0 to ignore the text content of that HTML element."

For TypeBoost there's no documentation yet, we could set this:
"Assign a boost of 0 to not score the item at all."