Fixing incorrect doc strings in control volume methods

[andrewlee94]

Fixes #793

Summary/Motivation:

Control volume docs strings stated that custom_term argument should be a Pyomo Expression when in fact they should be Python methods which return Pyomo expressions.

Changes proposed in this PR:

• Fix docs strings in CV0D and CV1D

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[Robbybp]

Thanks for fixing, looks good.

[codecov]

Codecov Report

Merging #820 (0de422c) into main (ebf51d2) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #820   +/-   ##
=======================================
  Coverage   67.91%   67.91%           
=======================================
  Files         567      567           
  Lines       62208    62208           
  Branches    11513    11513           
=======================================
+ Hits        42247    42251    +4     
  Misses      17784    17784           
+ Partials     2177     2173    -4     

Impacted Files	Coverage Δ
idaes/core/base/control_volume0d.py	89.15% <ø> (ø)
idaes/core/base/control_volume1d.py	88.24% <ø> (ø)
idaes/ver.py	61.53% <0.00%> (-4.62%)	⬇️
idaes/dmf/util.py	54.04% <0.00%> (+1.27%)	⬆️
idaes/dmf/codesearch.py	100.00% <0.00%> (+1.44%)	⬆️
idaes/conftest.py	90.90% <0.00%> (+6.06%)	⬆️

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