Docs: bad text wrapping

On the page http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/analysis-synonym-tokenfilter.html

even on a huge monitor the text is being wrapped the next way
```
mapping:
ipod, i-pod, i pod => ipod, i-pod, i pod
mapping:
ipod, i-pod, i pod => ipod
```

So one can think that "mapping:" is not in comment and is a part of syntax. But the lines are less than 80 chars, so perhaps the problem is in the page layout and there may be some other pages in the reference where the text is also being wrapped in an undesirable way.

Closes #7739
Update stemmer-tokenfilter.asciidoc

Change the `minimal_english` link to a publicly accessible URL
Tests: always run Lucene's CheckIndex when shards are closed in tests and fail the test if corruption is detected

Today we only run 10% of the time, and the test doesn't fail when
corruption is detected.

I think it's better to always run and fail the test, so we can catch
any possible resiliency bugs in Lucene/Elasticsearch causing corruption.

For known tests that create corrupted indices, it's easy to set
MockFSDirectoryService.CHECK_INDEX_ON_CLOSE to false...

Closes #7730
don't CheckIndex for this test case