Rollup merge of #106453 - coastalwhite:master, r=GuillaumeGomez

Improve include macro documentation

As outlined in #106118, the `include!` macro is a SEO problem when it comes to the Rust documentation. Beginners may see it as a replacement to `include` syntax in other languages. I feel like this documentation should quite explicitly link to the modules' documentation.

The primary goal of this PR is to address that issue by adding a warning to the documentation. While I was here, I also added some other parts. This included a `Uses` section and some (intra doc) links to other relevant topics.

I hope this can help beginners to Rust more quickly understand some multi-file project intricacies.

# References
- Syntax for the warning: https://github.com/tokio-rs/tracing/blob/58accc6da3f04af3f6144fbe6d68af7225c70c02/tracing/src/lib.rs#L55

Author: matthiaskrgr

library/core/src/macros/mod.rs

@@ -1315,22 +1315,41 @@ pub(crate) mod builtin {
 
     /// Parses a file as an expression or an item according to the context.
     ///
-    /// The file is located relative to the current file (similarly to how
-    /// modules are found). The provided path is interpreted in a platform-specific
-    /// way at compile time. So, for instance, an invocation with a Windows path
-    /// containing backslashes `\` would not compile correctly on Unix.
+    /// **Warning**: For multi-file Rust projects, the `include!` macro is probably not what you
+    /// are looking for. Usually, multi-file Rust projects use
+    /// [modules](https://doc.rust-lang.org/reference/items/modules.html). Multi-file projects and
+    /// modules are explained in the Rust-by-Example book
+    /// [here](https://doc.rust-lang.org/rust-by-example/mod/split.html) and the module system is
+    /// explained in the Rust Book
+    /// [here](https://doc.rust-lang.org/book/ch07-02-defining-modules-to-control-scope-and-privacy.html).
+    ///
+    /// The included file is placed in the surrounding code
+    /// [unhygienically](https://doc.rust-lang.org/reference/macros-by-example.html#hygiene). If
+    /// the included file is parsed as an expression and variables or functions share names across
+    /// both files, it could result in variables or functions being different from what the
+    /// included file expected.
+    ///
+    /// The included file is located relative to the current file (similarly to how modules are
+    /// found). The provided path is interpreted in a platform-specific way at compile time. So,
+    /// for instance, an invocation with a Windows path containing backslashes `\` would not
+    /// compile correctly on Unix.
     ///
-    /// Using this macro is often a bad idea, because if the file is
-    /// parsed as an expression, it is going to be placed in the
-    /// surrounding code unhygienically. This could result in variables
-    /// or functions being different from what the file expected if
-    /// there are variables or functions that have the same name in
-    /// the current file.
+    /// # Uses
+    ///
+    /// The `include!` macro is primarily used for two purposes. It is used to include
+    /// documentation that is written in a separate file and it is used to include [build artifacts
+    /// usually as a result from the `build.rs`
+    /// script](https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script).
+    ///
+    /// When using the `include` macro to include stretches of documentation, remember that the
+    /// included file still needs to be a valid rust syntax. It is also possible to
+    /// use the [`include_str`] macro as `#![doc = include_str!("...")]` (at the module level) or
+    /// `#[doc = include_str!("...")]` (at the item level) to include documentation from a plain
+    /// text or markdown file.
     ///
     /// # Examples
     ///
-    /// Assume there are two files in the same directory with the following
-    /// contents:
+    /// Assume there are two files in the same directory with the following contents:
     ///
     /// File 'monkeys.in':
     ///