Add how to mock scoped modules to Manual Mocks doc

[davidjb]

This adds info on how to mock scoped modules to the Manual Mocks docs page as it wasn't clear how this process would work.

(changelog modification pushing shortly once I have a PR ID)

[codecov-io]

Codecov Report

Merging #5638 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5638   +/-   ##
=======================================
  Coverage   62.28%   62.28%           
=======================================
  Files         216      216           
  Lines        7898     7898           
  Branches        3        4    +1     
=======================================
  Hits         4919     4919           
  Misses       2978     2978           
  Partials        1        1

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[github-actions]

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