r/datacenter: Update docs related to ESXi

Couple of things:

* Clarified the nature of the "default" datacenter on ESXi and how the
name attribute has no meaning.
* Moved all of this detail to the warning text.
* Have decided (not 100% related to this resource) that vsphere_host
will _require_ the vsphere_datacenter data source's output, even if
using ESXi. The purpose of these data sources and moving to using MoRefs
as the source of truth is to reduce ambiguity, and as such we shouldn't
be introducing ambiguity in.

website/docs/d/datacenter.html.markdown

@@ -9,9 +9,9 @@ description: |-
 # vsphere\_datacenter
 
 The `vsphere_datacenter` data source can be used to discover the ID of a
-vSphere datacenter. It can also be used to fetch the "default datacenter" on
-ESXi, however this can be done with [`vsphere_host`][data-source-vsphere-host]
-as well.
+vSphere datacenter. This can then be used with resources or data sources that
+require a datacenter, such as the [`vsphere_host`][data-source-vsphere-host]
+data source.
 
 [data-source-vsphere-host]: /docs/providers/vsphere/d/host.html
 
@@ -30,7 +30,10 @@ The following arguments are supported:
 * `name` - (String) The name of the datacenter. This can be a name or path.	If
   not provided, the default datacenter is used.
 
-~> **NOTE:** `name` is ignored on ESXi, and is not required.
+~> **NOTE:** When used against ESXi, this data source _always_ fetches the
+server's "default" datacenter, which is a special datacenter unrelated to the
+default datacenter that exists in vCenter. Hence, the `name` attribute is
+completely ignored.
 
 ## Attribute Reference