pgsql: Doc: improve index entry for "median".

Doc: improve index entry for "median".

We had an index entry for "median" attached to the percentile_cont function
entry, which was pretty useless because a person following the link would
never realize that that function was the one they were being hinted to use.

Instead, make the index entry point at the example in syntax-aggregates,
and add a <seealso> link to "percentile".

Also, since that example explicitly claims to be calculating the median,
make it use percentile_cont not percentile_disc. This makes no difference
in terms of the larger goals of that section, but so far as I can find,
nearly everyone thinks that "median" means the continuous not discrete
calculation.

Per gripe from Steven Winfield. Back-patch to 9.4 where we introduced
percentile_cont.

Discussion: https://postgr.es/m/20161223102056.25614.1166@wrigleys.postgresql.org

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a15c59d19624713f8e02fb11c5460003d9be98ed

Modified Files
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doc/src/sgml/func.sgml | 3 ---
doc/src/sgml/syntax.sgml | 14 ++++++++++++--
2 files changed, 12 insertions(+), 5 deletions(-)