[docs][msan] List common cases reported by msan (#101105)

Fixes google/sanitizers#1755.

Author: vitalybuka

clang/docs/MemorySanitizer.rst

@@ -8,11 +8,18 @@ MemorySanitizer
 Introduction
 ============
 
-MemorySanitizer is a detector of uninitialized reads. It consists of a
+MemorySanitizer is a detector of uninitialized memory use. It consists of a
 compiler instrumentation module and a run-time library.
 
 Typical slowdown introduced by MemorySanitizer is **3x**.
 
+Here is a not comprehensive of list cases when MemorySanitizer will report an error:
+
+* Uninitialized value was used in a conditional branch.
+* Uninitialized pointer was used for memory accesses.
+* Uninitialized value passed or returned from a function call, which is considered an undefined behavior. The check can be disabled with ``-fno-sanitize-memory-param-retval``.
+* Uninitialized data was passed into some libc calls.
+
 How to build
 ============