Add note about branch for custom instructions in CCR (#59942)

Author: hubwriter

content/copilot/how-tos/configure-custom-instructions/add-repository-instructions.md

@@ -182,6 +182,9 @@ Custom instructions are enabled for {% data variables.copilot.copilot_code-revie
 1. In the "Code & automation" section of the sidebar, click **{% octicon "copilot" aria-hidden="true" aria-label="copilot" %} {% data variables.product.prodname_copilot_short %}**, then **Code review**.
 1. Toggle the “Use custom instructions when reviewing pull requests” option on or off.
 
+> [!NOTE]
+> {% data reusables.copilot.code-review.custom-instructions-branch %}
+
 ## Further reading
 
 * [AUTOTITLE](/copilot/reference/custom-instructions-support)

content/copilot/tutorials/use-custom-instructions.md

@@ -43,10 +43,10 @@ Before writing custom instructions, it's helpful to understand how {% data varia
 * **Context limits**: Very long instruction files may result in some instructions being overlooked.
 * **Specificity matters**: Clear, specific instructions work better than vague directives.
 
-{% data reusables.copilot.code-review.custom-instructions-limit %}
-
 Keep these factors in mind as you write your instructions—they'll help you set realistic expectations and write more effective guidance.
 
+{% data reusables.copilot.code-review.custom-instructions-limit %}
+
 ## Writing effective custom instructions
 
 The key to successful custom instructions is to be clear, concise, and specific. Here are the core principles to follow:

data/reusables/copilot/code-review/custom-instructions-branch.md

@@ -0,0 +1 @@
+When reviewing a pull request, {% data variables.product.prodname_copilot_short %} uses the custom instructions in the base branch of the pull request. For example, if your pull request seeks to merge `my-feature-branch` into `main`, {% data variables.product.prodname_copilot_short %} will use the custom instructions in `main`.

data/reusables/copilot/code-review/custom-instructions-limit.md

@@ -1,2 +1,3 @@
 > [!NOTE]
-> {% data variables.copilot.copilot_code-review_short %} only reads the first 4,000 characters of any custom instruction file. Any instructions beyond this limit will not affect the reviews generated by {% data variables.copilot.copilot_code-review_short %}. This limit does not apply to {% data variables.copilot.copilot_chat_short %} or {% data variables.copilot.copilot_coding_agent %}.
+> * {% data variables.copilot.copilot_code-review_short %} only reads the first 4,000 characters of any custom instruction file. Any instructions beyond this limit will not affect the reviews generated by {% data variables.copilot.copilot_code-review_short %}. This limit does not apply to {% data variables.copilot.copilot_chat_short %} or {% data variables.copilot.copilot_coding_agent %}.
+> * {% data reusables.copilot.code-review.custom-instructions-branch %}