Help: Add notes for topic 'emulator-property'

Help/release/dev/emulator-property.rst

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+emulator-property
+-----------------
+
+* A :prop_tgt:`CROSSCOMPILING_EMULATOR` target property and supporting
+  :variable:`CMAKE_CROSSCOMPILING_EMULATOR` variable were introduced
+  to allow target platform binaries to run on the host during cross
+  compiling.