pgsql: Explain max_prepared_transactions requirement in isolation tests

Explain max_prepared_transactions requirement in isolation tests' README.

Now that we have a test that requires nondefault settings to pass, it seems
like we'd better mention that detail in the directions about how to run the
tests.

Also do some very minor copy-editing.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/11c88e59a60b2096ab42fa7cf9bff542717ec1e0

Modified Files
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src/test/isolation/README |   15 ++++++++++-----
1 files changed, 10 insertions(+), 5 deletions(-)