Discussion tickboxes for posts labeled as if for pages

[janeforshort]

When creating anew post, the box that allows you to turn on/off comments and pings says:
"Allow Comments on this page.
Allow trackbacks and pingbacks on this page."

(instead of "on this post.")

Not a big deal, but if an easy fix, would be good to fix to reduce confusion (the "I thought I was making a new post, did I hit Add New Page by accident?' thought).

Broken by changes for #6362 in [12228].

Didn't notice we were changing a shared meta box there.

(In [12323]) Revert the string change to the Discussion meta box as this is a generic box for multiple post types. Fixes #11346.