pgsql: In v10 release notes, call out sequence changes as a compatibili

In v10 release notes, call out sequence changes as a compatibility item.

The previous description didn't make it clear that this change
potentially breaks applications, partly because the entry wasn't even
in the compatibility-hazard section. Move and clarify.

Discussion: /messages/by-id/603f3f0a-f89d-ae8b-1da9-a92fac16086d@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/866f4a7c210857aa342bf901558d170325094dde

Modified Files
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doc/src/sgml/release-10.sgml | 66 +++++++++++++++++++++++++-------------------
1 file changed, 37 insertions(+), 29 deletions(-)

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