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.

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. $TESTDIR/testlib/pythonpath.sh

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cat >> $HGRCPATH << EOF

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[extensions]

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docgraph=

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EOF