Document the behaviour of underscore character '_' when filtering

[Jari Häkkinen]

The '_' character matches like the '.' character in regular expressions. This means that if a match for a true '_' character is wanted, then the combination "\_" must be used. A few examples in this would be nice.

[Nicklas Nordborg]

Pattern matching is only triggered if there is a % in the search string. But if there is also an underscore, this will match any character. I didn't know that it could be escaped with \. It seems to be a MySQL extension to SQL.