Add rustdoc doc page on how to write documentation

Author: GuillaumeGomez

src/doc/rustdoc/src/SUMMARY.md

@@ -1,6 +1,7 @@
 # The Rustdoc Book
 
 - [What is rustdoc?](what-is-rustdoc.md)
+- [How to write documentation](how-to-write-documentation.md)
 - [Command-line arguments](command-line-arguments.md)
 - [The `#[doc]` attribute](the-doc-attribute.md)
 - [Documentation tests](documentation-tests.md)

src/doc/rustdoc/src/how-to-write-documentation.md

@@ -0,0 +1,76 @@
+# How to write documentation
+
+This chapter covers not only how to write documentation but specifically
+how to write **good** documentation.  Something to keep in mind when
+writing documentation is that your audience is not just yourself but others
+who simply don't have the context you do.  It is important to be as clear
+as you can, and as complete as possible.  As a rule of thumb: the more
+documentation you write for your crate the better.  If an item is public
+then it should be documented.
+
+## Basic structure
+
+It is recommended that each item's documentation follows this basic structure:
+
+```text
+[short sentence explaining what it is]
+
+[more detailed explanation]
+
+[at least one code example that users can copy/paste to try it]
+
+[even more advanced explanations if necessary]
+```
+
+This basic structure should be straightforward to follow when writing your
+documentation and, while you might think that a code example is trivial,
+the examples are really important because they can help your users to
+understand what an item is, how it is used, and for what purpose it exists.
+
+Let's see an example coming from the [standard library] by taking a look at the
+[`std::env::args()`][env::args] function:
+
+``````text
+Returns the arguments which this program was started with (normally passed
+via the command line).
+
+The first element is traditionally the path of the executable, but it can be
+set to arbitrary text, and may not even exist. This means this property should
+not be relied upon for security purposes.
+
+On Unix systems shell usually expands unquoted arguments with glob patterns
+(such as `*` and `?`). On Windows this is not done, and such arguments are
+passed as-is.
+
+# Panics
+
+The returned iterator will panic during iteration if any argument to the
+process is not valid unicode. If this is not desired,
+use the [`args_os`] function instead.
+
+# Examples
+
+```
+use std::env;
+
+// Prints each argument on a separate line
+for argument in env::args() {
+    println!("{}", argument);
+}
+```
+
+[`args_os`]: ./fn.args_os.html
+``````
+
+As you can see, it follows the structure detailed above: it starts with a short
+sentence explaining what the functions does, then it provides more information
+and finally provides a code example.
+
+## Markdown
+
+`rustdoc` is using the [commonmark markdown specification]. You might be
+interested into taking a look at their website to see what's possible to do.
+
+[standard library]: https://doc.rust-lang.org/stable/std/index.html
+[env::args]: https://doc.rust-lang.org/stable/std/env/fn.args.html
+[commonmark markdown specification]: https://commonmark.org/