Add documentation for code owner branch protection (#245)

Documentation for https://github.com/go-gitea/gitea/pull/34995

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-on: https://gitea.com/gitea/docs/pulls/245
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Naxdy <naxdy@naxdy.org>

docs/usage/repository/code-owners.md

@@ -39,6 +39,9 @@ docs/(aws|google|azure)/[^/]*\\.(md|txt) @user8 @org1/team4
 !/assets/.*\\.(bin|exe|msi) @user9
 ```
 
+Code owners listed in this file will automatically be requested to review pull requests
+that modify files matching at least one corresponding rule.
+
 ### Escaping
 
 You can escape characters `#`, ` ` (space) and `\` with `\`, like:
@@ -55,3 +58,27 @@ Some character (`.+*?()|[]{}^$\`) should be escaped with `\\` inside regexp, lik
 path/\\.with\\.dots
 path/with\\+plus
 ```
+
+### Branch Protection Rules
+
+When setting up branch protection rules, you may choose to enforce code owner approval of
+modified files. This will block a PR from being merged until at least one code owner
+has given approval for every applicable rule listed in the `CODEOWNERS` file.
+
+For example, say you have the following `CODEOWNERS` file:
+
+```bash
+.*\\.go @Bob
+.*\\.md @Sally @Chris
+migrations/.* @Alice
+```
+
+This means that:
+
+- If your PR modifies any `.go` files, it will require approval from Bob.
+- If your PR modifies any `.md` files, it will require approval from _either_ Sally _or_ Chris.
+- If your PR modifies any file in the `migrations` directory, it will require approval from Alice.
+
+These rules are composable. So, for example, if your PR modifies some `.go` files, as well as
+some `.md` files, this means it will require approval from Bob, in addition to approval from
+one of either Sally or Chris.