pgsql: Polish the documentation concerning phrase text search.

Polish the documentation concerning phrase text search.

Fix grammar, improve examples, etc.

I did not attempt to document the current behavior concerning distance-zero
matches, because I think that's broken and needs to change, so I'm not
going to use up brain cells figuring out how to explain how it works now.
One way or the other, there's still more to write here.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/6581e930a8546a764e948ad429fc2e179fc38d09

Modified Files
--------------
doc/src/sgml/datatype.sgml | 17 ++--
doc/src/sgml/func.sgml | 25 ++++--
doc/src/sgml/textsearch.sgml | 179 +++++++++++++++++++++++++------------------
3 files changed, 136 insertions(+), 85 deletions(-)