Add info about the Spanish Plural Stemmer

[corradofiore]

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

Description

I am adding info about the undocumented Spanish Plural Stemmer (apache/lucene#11284, https://lucene.apache.org/core/9_11_0/analysis/common/org/apache/lucene/analysis/es/SpanishPluralStemmer.html)

Solution

I reworked the initial paragraph in the "Spanish" section and added an example.

Tests

No tests, just integrating the reference docs :-)

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[HoustonPutman]

This is awesome! Thanks so much for taking the time and also adding an example 🙂

[epugh]

LGTM. I am embarrassed that after encouraging Xavi to write the stemmer I didn't also push to have it be documented... @HoustonPutman are you merging or me?