[docs] mod_md documentation clarification

[Rich Bowen]

I'm reading the mod_md docs, and want some clarification on the 
following. In the docs for directive ManagedDomain, we have:

There are two special names that you may use in this directive: 'manual' 
and 'auto'. This determines if a Managed Domain shall have exactly the 
name list as is configured ('manual') or offer more convenience. With 
'auto' all names of a virtual host are added to a MD.

This is followed by an example that uses neither 'manual' nor 'auto', 
and thus it's unclear what the above paragraph refers to.

I believe that it refers to the MDDriveMode directive, which does not 
appear in the example.

I propose to patch the above paragraph to clarify that that's what's 
being talked about.

Related: I also would like to question the wisdom of having a 
ManagedDomain directive and a <ManagedDomain> container. This will most 
assuredly lead to user confusion and lengthy IRC conversations. I would 
request that the naming of one of these directives be reconsidered. (No, 
I'm not suggesting anything. I suck at naming things.)