Write the "passes" chapter of the rustdoc book

cc #42322

src/doc/rustdoc/src/passes.md

@@ -1,3 +1,86 @@
 # Passes
 
-Coming soon!
+Rustdoc has a concept called "passes". These are transformations that
+`rustdoc` runs on your documentation before producing its final output.
+
+In addition to the passes below, check out the docs for these flags:
+
+* [`--passes`](command-line-arguments.html#--passes-add-more-rustdoc-passes)
+* [`--no-defaults`](command-line-arguments.html#--no-defaults-dont-run-default-passes)
+
+## Default passes
+
+By default, rustdoc will run some passes, namely:
+
+* `strip-hidden`
+* `strip-private`
+* `collapse-docs`
+* `unindent-comments`
+
+However, `strip-private` implies `strip-private-imports`, and so effectively,
+all passes are run by default.
+
+## `strip-hidden`
+
+This pass implements the `#[doc(hidden)]` attribute. When this pass runs, it
+checks each item, and if it is annotated with this attribute, it removes it
+from `rustdoc`'s output.
+
+Without this pass, these items will remain in the output.
+
+## `unindent-comments`
+
+When you write a doc comment like this:
+
+```rust,ignore
+/// This is a documentation comment.
+```
+
+There's a space between the `///` and that `T`. That spacing isn't intended
+to be a part of the output; it's there for humans, to help separate the doc
+comment syntax from the text of the comment. This pass is what removes that
+space.
+
+The exact rules are left under-specified so that we can fix issues that we find.
+
+Without this pass, the exact number of spaces is preserved.
+
+## `collapse-docs`
+
+With this pass, multiple `#[doc]` attributes are converted into one single
+documentation string.
+
+For example:
+
+```rust,ignore
+#[doc = "This is the first line."]
+#[doc = "This is the second line."]
+```
+
+Gets collapsed into a single doc string of
+
+```text
+This is the first line.
+This is the second line.
+```
+
+## `strip-private`
+
+This removes documentation for any non-public items, so for example:
+
+```rust,ignore
+/// These are private docs.
+struct Private;
+
+/// These are public docs.
+pub struct Public;
+```
+
+This pass removes the docs for `Private`, since they're not public.
+
+This pass implies `strip-priv-imports`.
+
+## `strip-priv-imports`
+
+This is the same as `strip-private`, but for `extern crate` and `use`
+statements instead of items.