Searcher Manger with support for Multi-Reader

### Description

I'd like to use MultiReader inside my searcher manager, but currently there is only support for DirectoryReader. Not sure about the context for. why this was the case initially, seems specific to some desired use-case https://issues.apache.org/jira/browse/LUCENE-6087.

Anyway should be a 1-line change here: https://github.com/apache/lucene/blob/9e90fb5bf1807f1d9ccf2cabf74b3235f2431ed4/lucene/core/src/java/org/apache/lucene/search/SearcherManager.java#L125

I found a relevant stack overflow question here:
https://stackoverflow.com/questions/49817453/searchermanager-and-multireader-in-lucene

Start in lucene/core/src/java/org/apache/lucene/search/SearcherManager.java at the referenced line, where support is limited to DirectoryReader. Read the surrounding SearcherManager logic and the context in LUCENE-6087, then make the smallest change needed for MultiReader support. Done means SearcherManager accepts MultiReader as well as DirectoryReader.

Written by the indexing model from the issue text.