pgsql: Rearrange documentation paragraph describing pg_relation_size().

Rearrange documentation paragraph describing pg_relation_size().

Break the list of available options into an <itemizedlist> instead of
inline sentences.  This is mostly motivated by wanting to ensure that the
cross-references to the FSM and VM docs don't cross page boundaries in PDF
format; but it seems to me to read more easily this way anyway.  I took the
liberty of editorializing a bit further while at it.

Per complaint from Magnus about 9.0.18 docs not building in A4 format.
Patch all active branches so we don't get blind-sided by this particular
issue again in future.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/561ac1b8cd4ccc10f7c05e83b28ec87f5b38b20f

Modified Files
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doc/src/sgml/func.sgml |   40 ++++++++++++++++++++++++++++------------
1 file changed, 28 insertions(+), 12 deletions(-)