update markdown/help header format

Since gitiles recommends using # headers over ---/=== underlines,
change the manifest-format.md over and all our help texts.

Change-Id: I96391d41fba769e9f26870d497cf7cf01c8d8ab3
diff --git a/subcmds/forall.py b/subcmds/forall.py
index 693949e..6fb16f1 100644
--- a/subcmds/forall.py
+++ b/subcmds/forall.py
@@ -53,8 +53,7 @@
 The -r option allows running the command only on projects matching
 regex or wildcard expression.
 
-Output Formatting
------------------
+# Output Formatting
 
 The -p option causes '%prog' to bind pipes to the command's stdin,
 stdout and stderr streams, and pipe all output into a continuous
@@ -71,8 +70,7 @@
 causes command output to be suppressed until the command produces
 at least one byte of output on stdout.
 
-Environment
------------
+# Environment
 
 pwd is the project's working directory.  If the current client is
 a mirror client, then pwd is the Git repository.