Add known limitation to C416 with dictionaries (#13627)

Part of #13625

See also #13629

crates/ruff_linter/src/rules/flake8_comprehensions/rules/unnecessary_comprehension.rs

@@ -12,9 +12,8 @@ use crate::rules::flake8_comprehensions::fixes;
 /// Checks for unnecessary `dict`, `list`, and `set` comprehension.
 ///
 /// ## Why is this bad?
-/// It's unnecessary to use a `dict`/`list`/`set` comprehension to build a
-/// data structure if the elements are unchanged. Wrap the iterable with
-/// `dict()`, `list()`, or `set()` instead.
+/// It's unnecessary to use a `dict`/`list`/`set` comprehension to build a data structure if the
+/// elements are unchanged. Wrap the iterable with `dict()`, `list()`, or `set()` instead.
 ///
 /// ## Examples
 /// ```python
@@ -30,10 +29,33 @@ use crate::rules::flake8_comprehensions::fixes;
 /// set(iterable)
 /// ```
 ///
+/// ## Known problems
+///
+/// This rule may produce false positives for dictionary comprehensions that iterate over a mapping.
+/// The `dict` constructor behaves differently depending on if it receives a sequence (e.g., a
+/// `list`) or a mapping (e.g., a `dict`). When a comprehension iterates over the keys of a mapping,
+/// replacing it with a `dict` constructor call will give a different result.
+///
+/// For example:
+///
+/// ```pycon
+/// >>> d1 = {(1, 2): 3, (4, 5): 6}
+/// >>> {x: y for x, y in d1}  # Iterates over the keys of a mapping
+/// {1: 2, 4: 5}
+/// >>> dict(d1)               # Ruff's incorrect suggested fix
+/// (1, 2): 3, (4, 5): 6}
+/// >>> dict(d1.keys())        # Correct fix
+/// {1: 2, 4: 5}
+/// ```
+///
+/// When the comprehension iterates over a sequence, Ruff's suggested fix is correct. However, Ruff
+/// cannot consistently infer if the iterable type is a sequence or a mapping and cannot suggest
+/// the correct fix for mappings.
+///
 /// ## Fix safety
-/// This rule's fix is marked as unsafe, as it may occasionally drop comments
-/// when rewriting the comprehension. In most cases, though, comments will be
-/// preserved.
+/// Due to the known problem with dictionary comprehensions, this fix is marked as unsafe.
+///
+/// Additionally, this fix may drop comments when rewriting the comprehension.
 #[violation]
 pub struct UnnecessaryComprehension {
     obj_type: String,