pgsql: Document pg_get_multixact_members().

Document pg_get_multixact_members().

Oversight in commit 0ac5ad5134.

Author: Sami Imseih <samimseih@gmail.com>
Co-authored-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/20150619215231.GT133018%40postgresql.org
Discussion: https://postgr.es/m/CAA5RZ0sjQDDwJfMRb%3DZ13nDLuRpF13ME2L_BdGxi0op8RKjmDg%40mail.gmail.com
Backpatch-through: 13

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a167e654d06fa2ae7ba7e0f635b384cead8f6b7c

Modified Files
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doc/src/sgml/func.sgml        | 28 +++++++++++++++++++++++++++-
doc/src/sgml/maintenance.sgml |  5 ++++-
2 files changed, 31 insertions(+), 2 deletions(-)