Update agit-support.en-us.md (#27652)

Clarify Agit PR creation documentation

See https://github.com/go-gitea/gitea/issues/27579

docs/content/usage/agit-support.en-us.md

@@ -18,6 +18,7 @@ menu:
 # Agit Setup
 
 In Gitea `1.13`, support for [agit](https://git-repo.info/en/2020/03/agit-flow-and-git-repo/) was added.
+**Note**: git version 2.29 or higher is required on the server side for this to work.
 
 ## Creating PRs with Agit
 
@@ -32,14 +33,16 @@ git push origin HEAD:refs/for/main
 The command has the following structure:
 
 - `HEAD`: The target branch
-- `refs/<for|draft|for-review>/<branch>`: The target PR type
+- `origin`: The target repository (not a fork!)
+- `HEAD`: The local branch containing the changes you are proposing
+- `refs/<for|draft|for-review>/<branch>`: The target PR type and configuration
   - `for`: Create a normal PR with `<branch>` as the target branch
   - `draft`/`for-review`: Currently ignored silently
-- `<branch>/<session>`: The target branch to open the PR
+  - `<branch>/`: The branch you want your changes to be merged into
 - `-o <topic|title|description>`: Options for the PR
-  - `title`: The PR title
-  - `topic`: The branch name the PR should be opened for
-  - `description`: The PR description
+  - `topic`: The topic of this change. It will become the name of the branch holding the changes waiting for review.  This is REQUIRED to trigger a pull request.
+  - `title`: The PR title (optional but recommended), only used for topics not already having an associated PR.
+  - `description`: The PR description (optional but recommended), only used for topics not already having an associated PR.
   - `force-push`: confirm force update the target branch
 
 Here's another advanced example for creating a new PR targeting `main` with `topic`, `title`, and `description`: