Documentation: Note that MUST does not replace error propagation

Author: https://github.com/timschumi
Commit: https://github.com/SerenityOS/serenity/commit/03c3d7edbc
Pull-request: https://github.com/SerenityOS/serenity/pull/17187

Documentation/Patterns.md

@@ -63,19 +63,28 @@ Note: Our `TRY(...)` macro functions similarly to the `?` [operator in rust](htt
 The `MUST(...)` macro is similar to `TRY(...)` except the macro enforces that
 the code run inside the macro must succeed, otherwise we assert.
 
+Note that `MUST(...)` should not be used as a replacement for `TRY(...)` in cases where error propagation is not (currently) possible.
+Instead, the `release_value_but_fixme_should_propagate_errors()` method of `ErrorOr<>` should be used to retrieve the value
+and to mark the location for future improvement. `MUST(...)` is reserved for cases where we determine through other circumstances that it
+should not be possible for the code inside the macro to fail or if a failure is serious enough that the program _needs_ to crash.
+
 ### Example:
 
 ```cpp
-#include <AK/Try.h>
-#include <AK/String.h>
+#include <AK/Vector.h>
 
 ... snip ...
 
-void log_that_can_not_fail(StringView fmtstr, TypeErasedFormatParams& params)
+ErrorOr<void> insert_one_to_onehundred(Vector<int>& vector)
 {
-    StringBuilder builder;
-    MUST(vformat(builder, fmtstr, params));
-    return builder.to_deprecated_string();
+    TRY(vector.try_ensure_capacity(vector.size() + 100));
+
+    for (int i = 1; i <= 100; i++) {
+        // We previously made sure that we allocated enough space, so the append operation shouldn't ever fail.
+        MUST(vector.try_append(i));
+    }
+
+    return {};
 }
 ```