JP-1963: Add rshift fitgeom option to tweakreg docs

[hbushouse]

Closes #5841
Resolves JP-1963

Description

This PR updates the "tweakreg" step docs to add the "rshift" option to the list of allowed values for the "fitgeometry" param. It also expands the list of allowed values to add a brief description of the meaning of each value.

Checklist

• Tests
• Documentation
• Change log
• Milestone
• Label(s)

[codecov]

Codecov Report

Merging #5899 (70fe0cf) into master (a46bea8) will not change coverage.
The diff coverage is n/a.

❗ Current head 70fe0cf differs from pull request most recent head 9c016e2. Consider uploading reports for the commit 9c016e2 to get more accurate results

@@           Coverage Diff           @@
##           master    #5899   +/-   ##
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  Coverage   78.00%   78.00%           
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  Files         401      401           
  Lines       35285    35285           
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  Hits        27524    27524           
  Misses       7761     7761           

Flag	Coverage Δ		*Carryforward flag
nightly	78.00% <ø> (ø)		Carriedforward from a46bea8
unit	56.04% <ø> (ø)

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[hbushouse]

Updated rendered docs can be seen at https://jwst-pipeline--5899.org.readthedocs.build/en/5899/jwst/tweakreg/README.html

[cracraft]

The updates look good to me. Thanks.