Improving docs for strict_word_similarity()

While creating the release notes, I was confused by the description for
strict_word_similarity(), particularly "extent boundaries".  The
attached patch clarifies, at least for me, how word_similarity() and
strict_word_similarity() differ.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
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