pgsql: Doc: improve documentation of pg_signal_backend default role.

Doc: improve documentation of pg_signal_backend default role.

Give it an explanatory para like the other default roles have.
Don't imply that it can send any signal whatever.

In passing, reorder the table entries and explanatory paras
for the default roles into some semblance of consistency.

Ian Barwick, tweaked a bit by me.

Discussion: https://postgr.es/m/89907e32-76f3-7282-a89c-ea19c722fe5d@2ndquadrant.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e872432d629d9c4f0995a1e527dcddbcacbb7590

Modified Files
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doc/src/sgml/user-manag.sgml | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)