pgsql: Doc: minor improvement in pl/pgsql FETCH/MOVE documentation.

Doc: minor improvement in pl/pgsql FETCH/MOVE documentation.

Explain that you can use any integer expression for the "count" in
pl/pgsql's versions of FETCH/MOVE, unlike the SQL versions which only
allow a constant.

Remove the duplicate version of this para under MOVE. I don't see
a good reason to maintain two identical paras when we just said that
MOVE works exactly like FETCH.

Per Pavel Stehule, though I didn't use his text.

Discussion: https://postgr.es/m/CAFj8pRAcvSXcNdUGx43bOK1e3NNPbQny7neoTLN42af+8MYWEA@mail.gmail.com

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2e7a25c1dcb7acb9d1138934cade1f1a7e6fe690

Modified Files
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doc/src/sgml/plpgsql.sgml | 24 ++++--------------------
doc/src/sgml/ref/fetch.sgml | 2 +-
2 files changed, 5 insertions(+), 21 deletions(-)