pgsql: docs: Demote "Monitoring Disk Usage" from chapter to section.

docs: Demote "Monitoring Disk Usage" from chapter to section.

This chapter is very short, and the immediately preceding chapter is
called "Monitoring Database Activity". So, instead of having a
separate chapter for this, make it the last section of the preceding
chapter instead.

Discussion: http://postgr.es/m/CA+Tgmob7_uoYuS2=rVwpVXaRwP-UXz+++saYTC-BCZ42QzSNKQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f470b5c67924f791228e464b9c02cc1e53ef2906

Modified Files
--------------
doc/src/sgml/diskusage.sgml  | 144 -------------------------------------------
doc/src/sgml/filelist.sgml   |   1 -
doc/src/sgml/monitoring.sgml | 143 ++++++++++++++++++++++++++++++++++++++++++
doc/src/sgml/postgres.sgml   |   1 -
4 files changed, 143 insertions(+), 146 deletions(-)