docs: change `troubles` references to `instability` Per https://www.mercurial-scm.org/wiki/CEDVocabulary. There are a couple instances of 'troublesome' which probably need a word, as deriviatives of 'instability' didn't really fit. At least some of these are in the unlinked obs-terms.rst page.

. $TESTDIR/testlib/pythonpath.sh

cat >> $HGRCPATH << EOF
[extensions]
docgraph=
EOF