The termIndexInterval comment and example settings in the example solrconfig.xml file is left over from Solr 3.x versions. It does not apply to the default Solr 4.x installation and its presence in the example is confusing.
According to the JavaDocs for IndexWriterConfig, the Lucene level
implementations of setTermIndexInterval and setReaderTermsIndexDivisor these do not apply to the default Solr4 PostingsFormat implementation.
From (http://lucene.apache.org/core/4_10_0/core/org/apache/lucene/index/IndexWriterConfig.html#setTermIndexInterval%28int%29 )
"This parameter does not apply to all PostingsFormat implementations, including the default one in this release. It only makes sense for term indexes that are implemented as a fixed gap between terms. For example, Lucene41PostingsFormat implements the term index instead based upon how terms share prefixes. To configure its parameters (the minimum and maximum size for a block), you would instead use Lucene41PostingsFormat.Lucene41PostingsFormat(int, int). which can also be configured on a per-field basis:"
The (soon to be ) attached patch just removes the outdated example.
Documentation on the wiki and Solr ref guide should also be updated.
Since the Solr 4 default postingsformat is so much more efficient than Solr 3.x, there might no longer be a use case for messing with the parameters.
for users who have a strong need to explicitly control this type of setting, you'll need to do so by overridding the default codec – either by specifing a custom <codecFactory>, or by using the default SchemaCodecFactory, but specify per-field postingsFormats )(for the fields you want to modify) using custom classes – for example: that subclass Lucene41PostingsFormat using the desired values...