[docs] Use YARD deprecated to annotate treat_symbols_as_metadata_keys_with_true_values=

Created by: olleolleolle

This PR uses the YARD tag @deprecated in one more place.

Screenshots

This screenshot shows what the Methods search suggestions renders it as, and what the description of the method looks like.

This is what it looks like in the instance method summary (at the beginning of this page):

(The filter_run_when_matching in the description is a hyperlink.)