Aha! after more Internet searches and experimentation, it seems to be possible to search directory per directory – which is better than nothing – as easily as locating the directory and entering
*.doc "string" in the Search field.
As for "Add a filter", I quote from answers.microsoft.com:
Quote:
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"Add a search filter" is an invitation to use one of the filters in the line underneath it. It is not an invitation to set up filters of your own.
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An attitude strongly reminiscent of "any colour you like, as long as it's black" …