pgsql: Restructure foreign data wrapper chapter so it has more than one

Restructure foreign data wrapper chapter so it has more than one section.

As noted by Laurenz Albe, our SGML tools deal rather oddly with chapters
having just one <sect1>. Perhaps the tooling could be fixed, but really
the design of this chapter's introduction is pretty bogus anyhow. Split
it into a true introduction and a <sect1> about the FDW functions, so
that it reads better and dodges the lack-of-a-chapter-TOC problem.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/10323f3a592922bce75bc5c38589f0f3427ef232

Modified Files
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doc/src/sgml/fdwhandler.sgml | 83 +++++++++++++++++++++++-------------------
1 files changed, 45 insertions(+), 38 deletions(-)