BUG #15344: pg_proc.proisagg was removed incompatibly in PostgreSQL 11

The following bug has been logged on the website:

Bug reference:      15344
Logged by:          Lukas Eder
Email address:      lukas.eder@gmail.com
PostgreSQL version: 11beta2
Operating system:   Debian
Description:        

When comparing the current version (10) [1] and the developer version (11)
[2] of the pg_proc documentation, then it can be seen that the
pg_proc.proisagg column was removed backwards incompatibly. The
documentation states for [1]:

> The table contains data for aggregate functions as well as plain
functions. If proisagg is true, there should be a matching row in
pg_aggregate.

And for [2]:

> If prokind indicates that the entry is for an aggregate function, there
should be a matching row in pg_aggregate.

While I appreciate that an encoded enumeration is more powerful than a
simple boolean value, I think it would have been better if this change had
been done backwards compatibly. For example, there could still be a
pg_proc.proisagg column that defaults to (prokind = 'a').

[1] https://www.postgresql.org/docs/10/static/catalog-pg-proc.html
[2] https://www.postgresql.org/docs/11/static/catalog-pg-proc.html