Mention character limit for CIs in CCR (#58269)

Co-authored-by: mc <[email protected]>

Author: hubwriter

content/copilot/concepts/prompting/response-customization.md

@@ -12,7 +12,7 @@ redirect_from:
   - /copilot/concepts/code-review/coding-guidelines
   - /copilot/concepts/response-customization
 contentType: concepts
-category: 
+category:
   - Configure Copilot
 ---
 

data/reusables/copilot/repository-custom-instructions-support.md

@@ -20,3 +20,6 @@ The following table shows which {% data variables.product.prodname_copilot_short
 **4:** Repository-wide instructions, path-specific instructions, and agent instructions (using `AGENTS.md`, `CLAUDE.md` or `GEMINI.md` files).<br>
 **X:** Custom instructions are not supported.<br>
 **N/A:** Feature not available on this platform.
+
+> [!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 %}.