pgsql: Doc: call out UPDATE syntax change as a v10 compatibility issue.

Doc: call out UPDATE syntax change as a v10 compatibility issue.

The change made by commit 906bfcad7 means that if you're writing
a parenthesized column list in UPDATE ... SET, but that column list
is only one column, you now need to write ROW(expression) on the
righthand side, not just a parenthesized expression. This was an
intentional change for spec compatibility and potential future
expansion of the possibilities for the RHS, but I'd neglected to
document it as a compatibility issue, figuring that hardly anyone
would bother with parenthesized syntax for a single target column.
I was wrong, as shown by questions from Justin Pryzby, Adam Brusselback,
and others. Move the release note item into the compatibility section
and point out the behavior change for a single target column.

Discussion: https://postgr.es/m/CAMjNa7cDLzPcs0xnRpkvqmJ6Vb6G3EH8CYGp9ZBjXdpFfTz6dg@mail.gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7becb5fa1d8760ee70258ff23ce229ce5451c597

Modified Files
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doc/src/sgml/release-10.sgml | 46 +++++++++++++++++++++++++-------------------
1 file changed, 26 insertions(+), 20 deletions(-)