Help output: document that stop-on-fail implies trace

[tautschnig]

Running CBMC or JBMC with --stop-on-fail will print a counter-example
trace. Previously, the help output suggested that --trace was required
to accomplish this. Clarify in the help output that --trace is implied
by --stop-on-fail.

Fixes: #5654

• 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.
• 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 #5662 (b8287fe) into develop (f85b11d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5662   +/-   ##
========================================
  Coverage    69.41%   69.41%           
========================================
  Files         1243     1243           
  Lines       100621   100621           
========================================
  Hits         69850    69850           
  Misses       30771    30771           

Flag	Coverage Δ
cproversmt2	43.14% <ø> (ø)
regression	66.31% <ø> (ø)
unit	32.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
jbmc/src/jbmc/jbmc_parse_options.cpp	73.73% <ø> (ø)
src/cbmc/cbmc_parse_options.cpp	77.62% <ø> (ø)

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[martin-cs]

If we are keeping this behaviour then, yes, it should be documented.