Clarify help text on search page

[mnaydan]

testing notes

confirm updated help text is present on the archive search:

https://test-ppa.cdh.princeton.edu/archive/

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I would propose the following language, perhaps replacing the shadow text in the fields (this needs to be changed anyway since title is no longer just book title after adding excerpts)

Keyword or Phrase: "Search full-text and metadata, including approximate titles."

Title: "Search by exact title or subtitle."

Author: "Search by exact author (last name, first name)."

And then add, in the little blue help field: "Use quotes for exact phrase."

[mnaydan]

Looks good!