update diffIndexGenerate.ts

Author: MichaelCurrin

src/cli/diffIndexGenerate.ts

@@ -12,6 +12,8 @@ Check Git changes and generate a commit message.
 
 Options:
   --cached        Use only staged changes (equivalent to git --cached).
+    If the flag is omitted, then the standard \`git status\` logic is followed:
+    look for staged changes and use them, otherwise use unstaged changes.
   --help, -h      Show this help and exit.`;
 
 const DIFF_FLAGS = [