Auto merge of #12649 - flip1995:book-readme-sync, r=xFrednet

Consistent lint group table in book and README
The lint table and the restriction group description was improved in #10385, but only in the README. Apply the same changes to the book.
r? `@xFrednet`
I noticed that I left review comments about this in #10385, but never submitted them. So to this day they are listed as "pending" in the GitHub UI.
This is just copy and paste of the current README file on `master`.
changelog: none

Author: bors

book/src/README.md

@@ -18,17 +18,27 @@ category.
 |`clippy::all`| all lints that are on by default (correctness, suspicious, style, complexity, perf) |**warn/deny**|
 |`clippy::correctness`| code that is outright wrong or useless |**deny**|
 |`clippy::suspicious`| code that is most likely wrong or useless |**warn**|
+|`clippy::style`| code that should be written in a more idiomatic way |**warn**|
 |`clippy::complexity`| code that does something simple but in a complex way |**warn**|
 |`clippy::perf`| code that can be written to run faster |**warn**|
-|`clippy::style`| code that should be written in a more idiomatic way|**warn**|
-|`clippy::pedantic`| lints which are rather strict or might have false positives | allow |
+|`clippy::pedantic`| lints which are rather strict or have occasional false positives| allow|
+|`clippy::restriction`| lints which prevent the use of language and library features[^restrict]| allow |
 |`clippy::nursery`| new lints that are still under development | allow |
-|`clippy::cargo`| lints for the cargo manifest | allow || allow |
+|`clippy::cargo`| lints for the cargo manifest | allow |
+
+More to come, please [file an issue](https://github.com/rust-lang/rust-clippy/issues) if you have ideas!
+
+The `restriction` category should, *emphatically*, not be enabled as a whole. The contained
+lints may lint against perfectly reasonable code, may not have an alternative suggestion,
+and may contradict any other lints (including other categories). Lints should be considered
+on a case-by-case basis before enabling.
 
-More to come, please [file an
-issue](https://github.com/rust-lang/rust-clippy/issues) if you have ideas!
+[^restrict]: Some use cases for `restriction` lints include:
+- Strict coding styles (e.g. [`clippy::else_if_without_else`]).
+- Additional restrictions on CI (e.g. [`clippy::todo`]).
+- Preventing panicking in certain functions (e.g. [`clippy::unwrap_used`]).
+- Running a lint only on a subset of code (e.g. `#[forbid(clippy::float_arithmetic)]` on a module).
 
-The [lint list](https://rust-lang.github.io/rust-clippy/master/index.html) also
-contains "restriction lints", which are for things which are usually not
-considered "bad", but may be useful to turn on in specific cases. These should
-be used very selectively, if at all.
+[`clippy::else_if_without_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#else_if_without_else
+[`clippy::todo`]: https://rust-lang.github.io/rust-clippy/master/index.html#todo
+[`clippy::unwrap_used`]: https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_used