JP-2679: Update docs to reflect background subtraction update to acc…

[dmggh]

…ept rateints (3D) input

Resolves JP-2679

This PR updates the documentation for the code update for the background subtraction step to accept rateints (3D) input BKG exposures. The code update was PR #7049, which has been merged. Checklist for maintainers

• [ ] added entry in CHANGES.rst within the relevant release section
• [ ] updated or added relevant tests
• [X] updated relevant documentation
• [ ] added relevant milestone
• [ ] added relevant label(s)
• [ ] ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR
• [ ] Make sure the JIRA ticket is resolved properly

[codecov[bot]]

Codecov Report

Base: 79.72% // Head: 79.72% // No change to project coverage :thumbsup:

Coverage data is based on head (7f1628c) compared to base (d5f9a3f). Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7055   +/-   ##
=======================================
  Coverage   79.72%   79.72%           
=======================================
  Files         411      411           
  Lines       37270    37270           
=======================================
  Hits        29715    29715           
  Misses       7555     7555           

Flag	Coverage Δ		*Carryforward flag
nightly	79.71% <ø> (ø)		Carriedforward from 138d7b9
unit	52.03% <ø> (ø)

*This pull request uses carry forward flags. Click here to find out more.

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