Spring Document Tiles locale handling in definition filenames

In Tiles v3 integration, underscores in filenames are intended to identify the definition locale. This behavior is now documented in order to avoid unexpected results with filenames like tiles_definitions.xml.

This commit also updates Tiles v2 references to Tiles v3 in the Spring reference documentation.

Issue: SPR-11491

Comment From: rstoyanchev

This is now merged.