Unstable book documentation of tool lints

Author: flip1995

src/doc/unstable-book/src/language-features/tool-lints.md

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+# `tool_lints`
+
+The tracking issue for this feature is: [#44690]
+
+[#44690]: https://github.com./rust-lang/rust/issues/44690
+
+------------------------
+
+Tool lints let you use scoped lints, to `allow`, `warn`, `deny` or `forbid` lints of
+certain tools.
+
+Currently `clippy` is the only available lint tool.
+
+It is recommended for lint tools to implement the scoped lints like this:
+
+- `#[_(TOOL_NAME::lintname)]`: for lint names
+- `#[_(TOOL_NAME::lintgroup)]`: for groups of lints
+- `#[_(TOOL_NAME::all)]`: for (almost[^1]) all lints
+
+## An example
+
+```rust
+#![feature(tool_lints)]
+
+#![warn(clippy::pedantic)]
+
+#[allow(clippy::filter_map)]
+fn main() {
+    let v = vec![0; 10];
+    let _ = v.into_iter().filter(|&x| x < 1).map(|x| x + 1).collect::<Vec<_>>();
+    println!("No filter_map()!");
+}
+```
+
+[^1]: Some defined lint groups can be excluded here.