Contracts: document use of __CPROVER_loop_entry with arrays

[tautschnig]

Snapshotting of array-typed variables requires care as users may unintentionally copy the pointer rather than the contents.

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.22%. Comparing base (d2b4455) to head (095413f).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8470      +/-   ##
===========================================
- Coverage    78.31%   78.22%   -0.10%     
===========================================
  Files         1726     1726              
  Lines       188817   189038     +221     
  Branches     18270    18265       -5     
===========================================
  Hits        147874   147874              
- Misses       40943    41164     +221     

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