Update Lucene FAQ regarding index-time field boosting [LUCENE-4193]

Current FAQ says the following regarding index-time field boosts:

> Index time field boosts are worthless if you set them on every document\.

see [the FAQ](http://wiki.apache.org/lucene-java/LuceneFAQ#What_is_the_difference_between_field_.28or_document.29_boosting_and_query_boosting.3F).

I think, this should be changed to > Index time field boosts are worthless if you set them on every document _and solely search on this field at query time_\.

Because, when searching on _multiple_ fields, a match in a properly index-time boosted field will score higher than a match in a non-boosted field.

See [this discussion](https://forum.hibernate.org/viewtopic.php?f=9&t=1016615) on Hibernate Search forums.

Not sure if there are more places where similar statements are made.

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Migrated from [LUCENE-4193](https://issues.apache.org/jira/browse/LUCENE-4193) by Elmer van Chastelet

Open the linked Lucene FAQ and review the section on field or document boosting, then compare its statement with the issue's explanation and linked Hibernate Search discussion. Search the FAQ for similar wording if possible. Done means the inaccurate qualification is corrected and any directly related statement is consistent.

Written by the indexing model from the issue text.