lib/sh/util.sh: stderr_matches(): documented that the purpose of not redirecting fd 2 back to fd 2 is to allow log-filtering out an otherwise-confusing benign error

lib/sh/util.sh: ignore_err_msg(): documented that unlike `|| true`, this suppresses only errors caused by a particular error message, rather than all error exit statuses

lib/sh/util.sh: stdout_contains(): usage: documented that this requires a `{ ... } 41>&1` wrapper

lib/sh/util.sh: stderr2stdout(): clarified that fd 2 is not redirected back to fd 2

bugfix: lib/sh/util.sh: stderr_matches(): need to avoid redirecting stderr and stdout to the same place, because this prevents redirecting stdout back to the original stdout after stderr has been filtered using |

lib/sh/util.sh: local_export_array: renamed to just export_array because this was a replacement for export, not local_export

lib/sh/util.sh: local_*array: don't need -a because that it's an array is autodetected by the ()

lib/sh/util.sh: stderr_matches(): actually don't need to declare PIPESTATUS_ in a separate command, because local does support arrays

lib/sh/util.sh: added local_export_array

fix: lib/sh/util.sh: local_array: clarified that this is only needed for older versions of bash (the lack of support for arrays has apparently been fixed)