Improve .htaccess commenting

Done alongside improvements of the execution-pipeline.md docs.
Installer comment taken from d5723f7b0e

.htaccess

@@ -1,37 +1,44 @@
 ### SILVERSTRIPE START ###
+
+# Deny access to templates (but allow from localhost)
 <Files *.ss>
 	Order deny,allow
 	Deny from all
 	Allow from 127.0.0.1
 </Files>
 
+# Deny access to IIS configuration
 <Files web.config>
 	Order deny,allow
 	Deny from all
 </Files>
 
-# This denies access to all yml files, since developers might include sensitive
-# information in them. See the docs for work-arounds to serve some yaml files
+# Deny access to YAML configuration files which might include sensitive information
 <Files ~ "\.ya?ml$">
 	Order allow,deny
 	Deny from all
 </Files>
 
+# Route errors to static pages automatically generated by SilverStripe
 ErrorDocument 404 /assets/error-404.html
 ErrorDocument 500 /assets/error-500.html
 
 <IfModule mod_rewrite.c>
 	SetEnv HTTP_MOD_REWRITE On
-	RewriteEngine On	
-
+	RewriteEngine On
+	
+	# Deny access to potentially sensitive files and folders
 	RewriteRule ^vendor(/|$) - [F,L,NC]
 	RewriteRule silverstripe-cache(/|$) - [F,L,NC]
 	RewriteRule composer\.(json|lock) - [F,L,NC]
-
+	
+	# Process through SilverStripe if no file with the requested name exists.
+	# Pass through the original path as a query parameter, and retain the existing parameters.
 	RewriteCond %{REQUEST_URI} ^(.*)$
 	RewriteCond %{REQUEST_FILENAME} !-f
 	RewriteRule .* framework/main.php?url=%1 [QSA]
-
+	
+	# If framework isn't in a subdirectory, rewrite to installer
 	RewriteCond %{REQUEST_URI} ^(.*)/framework/main.php$
 	RewriteCond %{REQUEST_FILENAME} !-f
 	RewriteRule . %1/install.php? [R,L]