docs: Update documentation for new database user and dbname flags.
docs: Update documentation for new database user and dbname flags.

Updated the install documentation to include the explanation of the
two new install options `--postgresql-database-name` and
`--postgresql-database-user`.

hackerkid/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip