Document Git LFS and max file size interaction

Git LFS files are exempted from the maximum file size limit so that the
maximum file size limit can be used to force large files into Git LFS.

changelogs/unreleased/jramsay-clarify-git-lfs-push-rule-behavior.yml

+---
+title: Document Git LFS and max file size interaction
+merge_request: 17609
+author:
+type: other

doc/push_rules/push_rules.md

@@ -74,7 +74,7 @@ The following options are available.
 | Restrict by branch name | **Starter** 9.3 | Only branch names that match this regular expression are allowed to be pushed. Leave empty to allow any branch name. |
 | Restrict by commit author's email | **Starter** 7.10 | Only commit author's email that match this regular expression are allowed to be pushed. Leave empty to allow any email. |
 | Prohibited file names | **Starter** 7.10 | Any committed filenames that match this regular expression are not allowed to be pushed. Leave empty to allow any filenames. |
-| Maximum file size | **Starter** 7.12 | Pushes that contain added or updated files that exceed this file size (in MB) are rejected. Set to 0 to allow files of any size. |
+| Maximum file size | **Starter** 7.12 | Pushes that contain added or updated files that exceed this file size (in MB) are rejected. Set to 0 to allow files of any size. Files tracked by Git LFS are exempted. |
 
 TIP: **Tip:**
 GitLab uses [RE2 syntax](https://github.com/google/re2/wiki/Syntax) for regular expressions in push rules, and you can test them at the [GoLang regex tester](https://regex-golang.appspot.com).

ee/app/views/shared/push_rules/_form.html.haml

@@ -77,5 +77,6 @@
   .form-text.text-muted
     Pushes that contain added or updated files that exceed this file size are rejected.
     Set to 0 to allow files of any size.
+    Files tracked by Git LFS are exempted.
 
 = f.submit "Save Push Rules", class: "btn btn-success", data: { qa_selector: 'submit_settings_button' }