[clang] [LifetimeSafety] Update user documentation (PR #196790)

[Gábor Horváth via cfe-commits]

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@@ -265,8 +266,75 @@ it refers to has gone out of scope.
              p = &i; // OK!
            }
            (void)*p;
+          }
+
+The analysis also covers pointer arithmetic. If the result of pointer
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Xazax-hun wrote:

I am not sure if we need to call out supporting pointer arithmetic.  I'd drop this section. 

https://github.com/llvm/llvm-project/pull/196790