config: Consistent Markdown/Go wording for 'process'

I've dropped "main process" because "container process" is currently a
much more popular way of identifying that process in this
specification.  Before this commit:

  $ git grep -i 'main process' | wc -l
  4
  $ git grep -i 'container process' | wc -l
  13

I've also added our usual:

  (<type>, <required|optional>)

to the Markdown so folks can see that this is a required object.

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -90,6 +90,8 @@ See links for details about [mountvol](http://ss64.com/nt/mountvol.html) and [Se
 
 ## Process configuration
 
+**`process`** (object, required) configures the container process.
+
 * **`terminal`** (bool, optional) specifies whether you want a terminal attached to that process, defaults to false.
 * **`cwd`** (string, required) is the working directory that will be set for the executable.
   This value MUST be an absolute path.

specs-go/config.go

@@ -8,7 +8,7 @@ type Spec struct {
 	Version string `json:"ociVersion"`
 	// Platform specifies the configuration's target platform.
 	Platform Platform `json:"platform"`
-	// Process is the container's main process.
+	// Process configures the container process.
 	Process Process `json:"process"`
 	// Root is the root information for the container's filesystem.
 	Root Root `json:"root"`