docs(api): describe new drop_tip() behavior introduced in PAPI 2.18 (…

…#15078)

# Overview

Describe the new `apiLevel`-gated behavior for `drop_tip()` in Python
API 2.18.

Addresses RTC-448

# Test Plan

[Sandbox API ref
entry](http://sandbox.docs.opentrons.com/docstring-drop-tip-alternating-2.18/v2/new_protocol_api.html#opentrons.protocol_api.InstrumentContext.drop_tip)

# Changelog

Move ¶ about change in 2.15 down, and add another ¶ about 2.18.

# Review requests

This is how it works, right?

# Risk assessment

None, docstring only.

api/src/opentrons/protocol_api/instrument_context.py

@@ -1021,11 +1021,6 @@ def drop_tip(
         If no location is passed (e.g. ``pipette.drop_tip()``), the pipette will drop
         the attached tip into its :py:attr:`trash_container`.
 
-        Starting with API version 2.15, if the trash container is the default fixed
-        trash, the API will instruct the pipette to drop tips in different locations
-        within the trash container. Varying the tip drop location helps prevent tips
-        from piling up in a single location.
-
         The location in which to drop the tip can be manually specified with the
         ``location`` argument. The ``location`` argument can be specified in several
         ways:
@@ -1044,6 +1039,15 @@ def drop_tip(
               the ``WasteChute`` object. For example,
               ``pipette.drop_tip(location=waste_chute)``.
 
+        In API versions 2.15 to 2.17, if ``location`` is a ``TrashBin`` or not
+        specified, the API will instruct the pipette to drop tips in different locations
+        within the bin. Varying the tip drop location helps prevent tips
+        from piling up in a single location.
+
+        Starting with API version 2.18, the API will only vary the tip drop location if
+        ``location`` is not specified. Specifying a ``TrashBin`` as the ``location``
+        behaves the same as specifying :py:meth:`.TrashBin.top`, which is a fixed position.
+
         :param location:
             Where to drop the tip.