Improve .htaccess commenting

Done alongside improvements of the execution-pipeline.md docs. 

See https://github.com/silverstripe/silverstripe-installer/pull/73 for installer commit.

I've noticed the generated .htaccess file here doesn't have the last bit
in the default .htaccess file:

	RewriteCond %{REQUEST_URI} ^(.*)/framework/main.php$
 	RewriteCond %{REQUEST_FILENAME} !-f
 	RewriteRule . %1/install.php? [R,L]

It originated from @hafriedlander in 2011 (d5723f7), I'm not quite sure what it's trying to achieve.
Hamish, could you clarify?