doc: Replace "underline" with "## Section name".

This approach doesn't require variable-length underlines.

doc/HACKING/HowToReview.md

@@ -1,5 +1,4 @@
-How to review a patch
-=====================
+# How to review a patch
 
 Some folks have said that they'd like to review patches more often, but they
 don't know how.
@@ -9,9 +8,7 @@ So, here are a bunch of things to check for when reviewing a patch!
 Note that if you can't do every one of these, that doesn't mean you can't do
 a good review!  Just make it clear what you checked for and what you didn't.
 
-
-Top-level smell-checks
-----------------------
+## Top-level smell-checks
 
 (Difficulty: easy)
 
@@ -37,10 +34,9 @@ Top-level smell-checks
 - If this changes anything in the code, is there a "changes" file?
 
 
-Let's look at the code!
------------------------
+## Let's look at the code!
 
-- Does the code conform to CodingStandards.txt?
+- Does the code conform to `CodingStandards.md`?
 
 - Does the code leak memory?
 
@@ -60,8 +56,7 @@ Let's look at the code!
 - Is there duplicated code that could be turned into a function?
 
 
-Let's look at the documentation!
---------------------------------
+## Let's look at the documentation!
 
 - Does the documentation conform to CodingStandards.txt?
 
@@ -70,8 +65,7 @@ Let's look at the documentation!
 - Can you predict what the function will do from its documentation?
 
 
-Let's think about security!
----------------------------
+## Let's think about security!
 
 - If there are any arrays, buffers, are you 100% sure that they cannot
   overflow?