[ci skip] better instructions for patch apply conflict (#11568)

CONTRIBUTING.md

@@ -185,6 +185,8 @@ These steps assume the `origin` remote is your fork of this repository and `upst
 1. Checkout feature/fix branch and rebase on master: `git checkout patch-branch && git rebase master`.
 1. Apply updated patches: `./gradlew applyPatches`.
 1. If there are conflicts, fix them.
+   * If there are conflicts within `Paper-API`, after fixing them run `./gradlew rebuildApiPatches` before re-running `./gradlew applyPatches`.
+   * The API patches are applied first, so if there are conflicts in the API patches, the server patches will not be applied.
 1. If your PR creates new patches instead of modifying existing ones, in both the `Paper-Server` and `Paper-API` directories, ensure your newly-created patch is the last commit by either:
     * Renaming the patch file with a large 4-digit number in front (e.g. 9999-Patch-to-add-some-new-stuff.patch), and re-applying patches.
     * Running `git rebase --interactive base` and moving the commits to the end.