[docs] Add doc for pushdown_subfields_for_map_functions session property

[steveburnett]

Document the session property pushdown_subfields_for_map_functions added in #25451.

Expected Behavior or Use Case

Documentation for the session property pushdown_subfields_for_map_functions should exist in the Presto documentation.

Presto Component, Service, or Connector

Documentation.

Possible Implementation

Add doc for session property pushdown_subfields_for_map_functions to admin/properties-session.rst.

Context

While reviewing #25623 I noticed that #25451 lacked documentation for the session property pushdown_subfields_for_map_functions that it introduced.