pgsql: doc: Database and tablespace size functions throws error

doc: Database and tablespace size functions throws error

Commit b6463ea6ef3e46 added an explicit ereport on missing object
to pg_database_size and pg_tablespace_size, but the documentation
wasn't updated to match.

Author: Zhong ShiHao <zhong950419(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: solai v <solai(dot)cdac(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGRkXqQSegqDnkRf344G=80mJMn78o9u=auHnx=u1jf=XpuE0A@mail.gmail.com
Backpatch-through: 18

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2874526137968132731a2fbfe1b779353a488c1c

Modified Files
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doc/src/sgml/func/func-admin.sgml | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)