pgsql: Make sure that 'make distclean' cleans out all files that are not

Log Message:
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Make sure that 'make distclean' cleans out all files that are not meant
to be shipped. Also, allow 'make clean' to remove intermediate working files.

Modified Files:
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pgsql/doc/src/sgml:
Makefile (r1.125 -> r1.126)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.125&r2=1.126)