pgsql: Improve some documentation about the bootstrap superuser.

Improve some documentation about the bootstrap superuser.

This commit adds some notes about the inability to remove superuser
privileges from the bootstrap superuser.  This has been blocked
since commit e530be2c5c, but it wasn't intended be a supported
feature before that, either.

In passing, change "bootstrap user" to "bootstrap superuser" in a
couple places.

Author: Yurii Rashkovskii
Reviewed-by: Vignesh C, David G. Johnston
Discussion: https://postgr.es/m/CA%2BRLCQzSx_eTC2Fch0EzeNHD3zFUcPvBYOoB%2BpPScFLch1DEQw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d891dcc065a2b590102ec75121828fed97acf9ba

Modified Files
--------------
doc/src/sgml/glossary.sgml       | 3 ++-
doc/src/sgml/ref/alter_role.sgml | 4 +++-
doc/src/sgml/user-manag.sgml     | 2 +-
src/backend/commands/user.c      | 2 +-
4 files changed, 7 insertions(+), 4 deletions(-)