goto-diff: include --verbosity in help output

[tautschnig]

Add missing command-line option documentation for --verbosity.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6920 (0827860) into develop (e42a46d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6920   +/-   ##
========================================
  Coverage    77.78%   77.79%           
========================================
  Files         1568     1568           
  Lines       180269   180301   +32     
========================================
+ Hits        140230   140262   +32     
  Misses       40039    40039           

Impacted Files	Coverage Δ
src/goto-diff/goto_diff_parse_options.cpp	60.22% <ø> (ø)
src/util/config.cpp	57.85% <100.00%> (ø)
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	99.66% <0.00%> (+0.01%)	⬆️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	87.80% <0.00%> (+0.10%)	⬆️

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