pgsql: Doc: Restructure B-Tree support routine docs.

Doc: Restructure B-Tree support routine docs.

Use a top-level "variablelist", with one item per B-Tree support
function. This structure matches the structure used by various
"Extensibility" sections in other documentation chapters for other index
access methods.

An explicit list makes it much clearer where each item begins and ends.
This wasn't really a problem before now, but an upcoming patch that adds
deduplication to nbtree will need to have its own new B-Tree support
function. Ease the burden of translators by tidying up btree.sgml ahead
of committing the deduplication patch.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/caba0910afa124b8c6c61208b487846eea6b1970

Modified Files
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doc/src/sgml/btree.sgml | 420 ++++++++++++++++++++++++++----------------------
1 file changed, 225 insertions(+), 195 deletions(-)