pgsql: Update type-conversion documentation for long-ago changes.

Update type-conversion documentation for long-ago changes.

This example wasn't updated when we changed the behavior of bpcharlen()
in 8.0, nor when we changed the number of parameters taken by the bpchar()
cast function in 7.3.  Per report from lsliang.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b7f83e70d9758c5dd480c9dbba1668ddeec37916

Modified Files
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doc/src/sgml/typeconv.sgml |   22 ++++++++++++----------
1 files changed, 12 insertions(+), 10 deletions(-)