Document state of smt2_incremental_decision_proceduret

[thomasspriggs]

This PR documents the state of smt2_incremental_decision_proceduret.
This is useful because the state has grown large enough that saying a bit more about the purpose of each piece of state will help with maintaining it.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[TGWDB]

Great!

[codecov]

Codecov Report

Merging #7062 (d7b3f48) into develop (8258529) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #7062      +/-   ##
===========================================
- Coverage    77.88%   77.86%   -0.03%     
===========================================
  Files         1569     1569              
  Lines       180995   180995              
===========================================
- Hits        140968   140929      -39     
- Misses       40027    40066      +39     

Impacted Files	Coverage Δ
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	82.18% <0.00%> (-6.38%)	⬇️

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