Document SELECT location

[blais]

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Original report by Martin Michlmayr (Bitbucket: tbm13, GitHub: tbm).

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Martin mentioned on the list recently:

one can just use a SQL command, something like SELECT location WHERE date >= 2019-02-01 and flag ='!' and iterate using "next-error" in Emacs. Perhaps that should be documented better.

Zack said it's not mentioned in the BQL documentation.