Search indexing calls are inefficient

Problem/Motivation

On #3075695: Move search_index() and related functions to a service, in the last couple of hours before it was committed, a change was rushed through that left Search module indexing in an inefficient state.

This should be fixed before the 8.8 release IMO.

Two problems:

a) The plugins that are calling ->index() are not passing the $needs_update in as FALSE, but they are still calling updateWordWeights(). For example, here is HelpSearch from that patch:

+            $words += $this->searchIndex->index($this->getType(), $item->sid, $langcode, $text);
+          }
         }
       }
     }
+    finally {
+      $this->searchIndex->updateWordWeights($words);
+    }

So HelpSearch is keeping track of $words and calling updateWordWeights, but because it didn't pass in FALSE as the last argument, the ->index() function is updating itself:

+  public function index($type, $sid, $langcode, $text, $update_weights = TRUE) {
...
+    finally {
+      if ($update_weights) {
+        $this->updateWordWeights($current_words);
+      }
+    }

So, updateWordWeights() is being called on every index() call, and then again at the end. Inefficient and redundant.

b) The docs in index() telling you what function to call when you do pass in FALSE were removed in one of the interdiffs above. So people looking at SearchIndexInterface() don't have an explicit message that if they do pass in FALSE saying how they should update the word weights later.

Proposed resolution

a) Fix HelpSearch and NodeSearch to do the right thing with calling index() so it is more efficient.
b) Update the docs on SearchIndexInterface

Remaining tasks

Make a patch.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None needed.