tracker-needle: Update tooltip for category searching to mention FTS
tracker-needle: Update tooltip for category searching to mention FTS

Basically, it's not clear that this approach searches the content of files
too. It is clear from the other categories that this isn't the case.

This fixes GB#686071.

hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker