docs: add a note to injecting multiple devices docs

[noemifrisina]

From the documentation page it wasn't obvious that the DeviceComposite would need to be imported into the same planFunctions module that will need to make use of it, so adding a small note.
This led to multiple pydantic errors on the beamline that did not help understanding what the actual problem was (issue tbd)

[rtuck99]

Suggested change

	> In order for this to work, the `MyDeviceComposite` needs to be mae available to blueapi by importing it into the `planFunctions` module where it's needed.
	> In order for this to work, the `MyDeviceComposite` needs to be made available to blueapi by importing it into the `planFunctions` module where it's needed. Factories for the individual devices within the composite also need to be available in the relevant beamline device module.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.30%. Comparing base (8820bbe) to head (8462f9d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1525   +/-   ##
=======================================
  Coverage   95.30%   95.30%           
=======================================
  Files          43       43           
  Lines        3198     3198           
=======================================
  Hits         3048     3048           
  Misses        150      150           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.