[docs] should mention that filter only supports XPath 1.0

[kares]

XPath 1.0 is pretty outdated these days, unfortunately newer versions are not supported.
Nokogiri is currenlty based on Xalan, which hasn't been updated for a while and only supports 1.0

Plugin's (4.0.7) doc links to a general XPath page which mentions all versions
(so its easy to assume all XPath functions work) :

XPath 1.0 became a W3C Recommendation on November 16, 1999.

XPath 2.0 became a W3C Recommendation on January 23, 2007.

XPath 3.0 became a W3C Recommendation on April 8, 2014.