Reword documentation of __CPROVER_{r,w,rw}_ok

[tautschnig]

It is safe to use these primitives for asserting validity of pointers.

Fixes: #8217, #8199

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.22%. Comparing base (a209b44) to head (cfc3277).
Report is 33 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #8472    +/-   ##
=========================================
  Coverage    78.22%   78.22%            
=========================================
  Files         1726     1726            
  Lines       189038   189038            
  Branches     18265    18467   +202     
=========================================
  Hits        147874   147874            
  Misses       41164    41164            

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[celinval]

I think it also returns false if p is NULL.

Is there a reason why we don't want to say that this would return a non-deterministic value otherwise?