DOC: Update git message guidelines

docs/guidelines/git.rst

@@ -104,7 +104,8 @@ Subject
 
 Please follow these rules for a good subject:
 
-- Limit the subject to **72 characters**, as it will be truncated by different systems
+- Limit the **summary** (aka subject) to **50 characters**
+- Limit the **description** (aka body) to **72 characters**
 - **Capitalize** the sentence
 - Do not end the sentence with a **period**, as it's unnecessary
 - Use the **imperative** mood