pgsql: doc: Clarify where ereport severity levels are defined

doc: Clarify where ereport severity levels are defined

For a reader unfamiliar with the postgres code it might take some
grepping to find where elevels are defined. This adds a reference
to elog.h in the text like how SQLSTATE errorcodes are referenced
to errcodes.h on the same page.

Author: Kuwamura Masaki <kuwamura@db.is.i.nagoya-u.ac.jp>
Discussion: https://postgr.es/m/CAMyC8qqp1UDA9zothnJ9CbUYByytwpALS3LkdZ6bs1w5kZw5Xg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/22ff5c9d78b9c23622de2cf0730cec85266a7596

Modified Files
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doc/src/sgml/sources.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)