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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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/db3cf2529304f96eabedd21f292bc661d5772aee

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