[CXX Safe Buffer] Update the documentation for unsafe_buffer_usage attribute (llvm#135087)

Update the documentation for the unsafe_buffer_usage attribute to
capture the new behavior introduced by
llvm#125671

Co-authored-by: MalavikaSamak <[email protected]>

Author: malavikasamak

clang/include/clang/Basic/AttrDocs.td

@@ -7517,9 +7517,11 @@ the field it is attached to, and it may also lead to emission of automatic fix-i
 hints which would help the user replace the use of unsafe functions(/fields) with safe
 alternatives, though the attribute can be used even when the fix can't be automated.
 
-* Attribute attached to functions: The attribute does not suppress
-  ``-Wunsafe-buffer-usage`` inside the function to which it is attached.
-  These warnings still need to be addressed.
+* Attribute attached to functions: The attribute suppresses all
+  ``-Wunsafe-buffer-usage`` warnings within the function it is attached to, as the
+  function is now classified as unsafe. The attribute should be used carefully, as it
+  will silence all unsafe operation warnings inside the function; including any new
+  unsafe operations introduced in the future.
 
   The attribute is warranted even if the only way a function can overflow
   the buffer is by violating the function's preconditions. For example, it