Remove trailing whitespace from *.sgml files.

Historically we've been lax about this, but seeing that we're not
lax in C files, there doesn't seem to be a good reason to be so
in the documentation.  Remove the existing occurrences (mostly
though not entirely in copied-n-pasted psql output), and modify
.gitattributes so that "git diff --check" will warn about future
cases.

While at it, add *.pm to the set of extensions .gitattributes
knows about, and remove some obsolete entries for files that
we don't have in the tree anymore.

Per followup discussion of commit 5a892c9b1.

Discussion: https://postgr.es/m/E1nfcV1-000kOR-E5@gemulon.postgresql.org

.gitattributes

@@ -1,8 +1,9 @@
 *		whitespace=space-before-tab,trailing-space
 *.[chly]	whitespace=space-before-tab,trailing-space,indent-with-non-tab,tabwidth=4
 *.pl		whitespace=space-before-tab,trailing-space,tabwidth=4
+*.pm		whitespace=space-before-tab,trailing-space,tabwidth=4
 *.po		whitespace=space-before-tab,trailing-space,tab-in-indent,-blank-at-eof
-*.sgml		whitespace=space-before-tab,trailing-space,tab-in-indent,-blank-at-eol
+*.sgml		whitespace=space-before-tab,trailing-space,tab-in-indent
 *.x[ms]l	whitespace=space-before-tab,trailing-space,tab-in-indent
 
 # Avoid confusing ASCII underlines with leftover merge conflict markers
@@ -17,9 +18,6 @@ src/backend/utils/Gen_dummy_probes.pl.prolog	whitespace=-blank-at-eof
 
 # Test output files that contain extra whitespace
 *.out					-whitespace
-contrib/*/output/*.source		-whitespace
-src/pl/plpgsql/src/output/*.source	-whitespace
-src/test/regress/output/*.source	-whitespace
 src/interfaces/ecpg/test/expected/*	-whitespace
 src/interfaces/libpq/test/expected.out	whitespace=-blank-at-eof
 
@@ -27,7 +25,6 @@ src/interfaces/libpq/test/expected.out	whitespace=-blank-at-eof
 configure				-whitespace
 ppport.h				-whitespace
 src/backend/regex/COPYRIGHT		-whitespace
-src/backend/regex/re_syntax.n		-whitespace
 src/backend/snowball/libstemmer/*.c	-whitespace
 src/backend/utils/mb/Unicode/*-std.txt	-whitespace
 src/include/snowball/libstemmer/*	-whitespace

doc/src/sgml/ref/pgarchivecleanup.sgml

@@ -161,7 +161,6 @@ pg_archivecleanup:  removing file "archive/00000001000000370000000E"
   </para>
  </refsect1>
 
- 
  <refsect1>
   <title>Notes</title>