[docs] Add a note on non-deterministic sorting order of equal elements

[mgrang]

Reviewers, if you do not have any problem with the wording of the note I would like to commit this.

[RKSimon]

LGTM

llvm/trunk/docs/CodingStandards.rst

 might not necessarily be "wrong code", this non-determinism might result
 in unexpected runtime crashes or simply hard to reproduce bugs on the
 customer side making it harder to debug and fix.
 
 As a rule of thumb, in case an ordered result is expected, remember to
 sort an unordered container before iteration. Or use ordered containers
 like vector/MapVector/SetVector if you want to iterate pointer keys.
 
+Beware of non-deterministic sorting order of equal elements
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+std::sort uses a non-stable sorting algorithm in which the order of equal
+elements is not guaranteed to be preserved. Thus using std::sort for a
+container having equal elements may result in non-determinstic behavior.
+To uncover such instances of non-determinism, LLVM has introduced a new
+llvm::sort wrapper function. For an EXPENSIVE_CHECKS build this will randomly
+shuffle the container before sorting. As a rule of thumb, always make sure to
+use llvm::sort instead of std::sort.
+
 Style Issues
 ============
 
 The High-Level Issues
 ---------------------
 
 Self-contained Headers
 ^^^^^^^^^^^^^^^^^^^^^^