List supported coverage criteria in CBMC's help output

[tautschnig]

Rather than making the user read further documentation, list all
(currently) supported coverage criteria when running cbmc --help.

For consistency, also add "assumes" as an alternative way to request
coverage of assume statements (previously only "assume" was supported,
even though we otherwise included the plural variants where applicable).

• 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.

[peterschrammel]

except for mcdc and cover... I'd drop the mention of plural.
Has path ever been implemented?

[tautschnig]

Thanks, I've updated the text. Also, I thus removed "path" as it seems the only "implementation" is a "TODO" ...

[codecov]

Codecov Report

Merging #6642 (b17a993) into develop (216e74e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6642   +/-   ##
========================================
  Coverage    76.72%   76.72%           
========================================
  Files         1579     1579           
  Lines       181938   181938           
========================================
  Hits        139587   139587           
  Misses       42351    42351           

Impacted Files	Coverage Δ
src/goto-instrument/cover.cpp	83.93% <100.00%> (ø)

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