Use the new section title style in Asciidoctor.

We previous use the section title style like:

Section level 1
===============

Section level 2
---------------

Which have a problem in Asciidoctor that the number of "="s or "-"s must match
the number of characters in the header exactly, as a result it's easy to make
mistakes while changing the titles. Asciidoctor provides a better style like:

= Section level 1

== Section level 2

So we switched to this style.

Also fixed a bug in replace_macros.py, which will not cause any problem in the
old style.

Change-Id: I811dd7238735d98f662767c17086152cd69aea02
diff --git a/Documentation/error-multiple-changeid-lines.txt b/Documentation/error-multiple-changeid-lines.txt
index 9b094d4..ca15270 100644
--- a/Documentation/error-multiple-changeid-lines.txt
+++ b/Documentation/error-multiple-changeid-lines.txt
@@ -1,5 +1,4 @@
-multiple Change-Id lines in commit message footer
-=================================================
+= multiple Change-Id lines in commit message footer
 
 With this error message Gerrit rejects to push a commit if the commit
 message footer of the pushed commit contains several Change-Id lines.
@@ -20,8 +19,7 @@
 will be automatically generated and inserted.
 
 
-SEE ALSO
---------
+== SEE ALSO
 
 * link:user-changeid.html[Change-Id Lines]