doc: improve commit messages

Fixes #24852

Change-Id: I71d0d7f75108bf4ad606733a45bb71baa66a4e91
Reviewed-on: https://go-review.googlesource.com/107197Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Giovanni Bajo <rasky@develer.com>

doc/contribute.html

@@ -579,13 +579,14 @@ The algorithm is described at http://wikipedia.org/wiki/McGillicutty_Algorithm
 
 Fixes #159
 </pre>
+
 </p>
 
 <h3>First line</h3>
 
 <p>
 The first line of the change description is conventionally a one-line
-summary of the change, prefixed by the primary affected package.</p>
+summary of the change, prefixed by the primary affected package. Keep the first line as short as possible (ideally, under 76 characters).</p>
 
 <p>It should be written so to complete the sentence "This change modifies Go to _____."</p>
 
@@ -595,7 +596,7 @@ summary of the change, prefixed by the primary affected package.</p>
 change and explain what it does.
 Write in complete sentences with correct punctuation, just like
 for your comments in Go.
-If there is a helpful reference, mention it here.
+No Markdown should be used.
 </p>
 
 <h3>Referencing issues</h3>
@@ -621,10 +622,6 @@ linked to the issue in the main repository. The correct form is "Fixes golang/go
 </p>
 
 
-
-
-
-
 <h2 id="review">The review process</h2>
 
 <p>