Align legacy issue_write body description with improved text

Author: jluocsa

pkg/github/__toolsnaps__/issue_write_ff_remote_mcp_issue_fields.snap

@@ -22,7 +22,7 @@
         "type": "array"
       },
       "body": {
-        "description": "Issue body content",
+        "description": "Issue body content (Markdown). On `method=\"update\"`, this REPLACES the entire issue body; use `add_issue_comment` to add a comment without modifying the body.",
         "type": "string"
       },
       "duplicate_of": {

pkg/github/issues.go

@@ -2119,7 +2119,7 @@ Options are:
 					},
 					"body": {
 						Type:        "string",
-						Description: "Issue body content",
+						Description: "Issue body content (Markdown). On `method=\"update\"`, this REPLACES the entire issue body; use `add_issue_comment` to add a comment without modifying the body.",
 					},
 					"assignees": {
 						Type:        "array",