Improve documentation for imports_merge_style

Author: goffrie

Configurations.md

@@ -1709,14 +1709,14 @@ pub enum Foo {}
 
 Merge together related imports based on their paths.
 
-This option requires `reorder_imports`, which is enabled by default.
-
 - **Default value**: `Preserve`
 - **Possible values**: `Preserve`, `Crate`, `Module`
-- **Stable**: No (tracking issue: [#3362](https://github.com/rust-lang/rustfmt/issues/3362))
+- **Stable**: No
 
 #### `Preserve` (default):
 
+Do not perform any merging and preserve the original structure written by the developer.
+
 ```rust
 use foo::b;
 use foo::b::{f, g};
@@ -1726,6 +1726,8 @@ use qux::{h, i};
 
 #### `Crate`:
 
+Merge imports from the same crate into a single `use` statement. Conversely, imports from different crates are split into separate statements.
+
 ```rust
 use foo::{
     a, b,
@@ -1738,6 +1740,8 @@ use qux::{h, i};
 
 #### `Module`:
 
+Merge imports from the same module into a single `use` statement. Conversely, imports from different modules are split into separate statements.
+
 ```rust
 use foo::b::{f, g};
 use foo::d::e;

tests/source/imports_merge_style_crate.rs

@@ -32,3 +32,6 @@ use g::{self, b};
 use h::{a};
 use i::a::{self};
 use j::{a::{self}};
+
+use {k::{a, b}, l::{a, b}};
+use {k::{c, d}, l::{c, d}};

tests/target/imports_merge_style_crate.rs

@@ -23,3 +23,6 @@ use g::{self, a, b};
 use h::a;
 use i::a::{self};
 use j::a::{self};
+
+use k::{a, b, c, d};
+use l::{a, b, c, d};