Add and document a nuxeo.conf parameter to set fulltext limit size

Since NXP-20872 there is a size limit in the indexed fulltext for a document.

The default for fieldSizeLimit is 128KB.

On average this can limit the indexed text to the first 45 pages of a text document.

The text above this limit will not be searchable and it is hard to find out why and correct the problem.

Proposed improvements:

• make the limit configurable from nuxeo.conf instead of having to override the big default-repository-config contribution.
• document the limit
• log at info level instead of debugging level when full text is truncated (org.nuxeo.ecm.core.storage.FulltextExtractorWork#limitStringSize)