pgsql: Add comments about the need to avoid uninitialized bits in datat

Add comments about the need to avoid uninitialized bits in datatype values.

There was already one recommendation in the documentation about writing
C functions to ensure padding bytes are zeroes, but make it stronger.

Also fix an example that was still using direct assignment to a varlena
length word, which no longer works since the varvarlena changes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4f6c75b541385eb2d48f7ef62c1c323ec2642134

Modified Files
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doc/src/sgml/xfunc.sgml | 26 ++++++++++++++++++++------
1 files changed, 20 insertions(+), 6 deletions(-)