feat(GlFilteredSearch): Clarify use of text values

What does this MR do?

Clarifies that filtered search component can accept "user input", not just dropdown selections (which themselves become tokenized text input).

To accommodate a wider range of possible fields to filter on it will be beneficial in limited cases to allow text values not assigned from a dropdown selector. The first example of this is with date fields as in gitlab-org/gitlab!120160 (merged). While this could benefit from a datepicker in a later iteration, even with the datepicker users would expect to be able to type their date and a datepicker won't offer the same "dropdown" style selection as it wouldn't function like a combobox.

It is also possible already today to apply a search with text that has not been selected from a dropdown, which becomes tokenized input.

Original suggestion from @danmh

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