Combine and simplify strings

[Presskopp]

We made the "unknown post type" invalid, see #39171

We could take this a step further and change

unknown to invalid in:

"Unknown Feed"
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/widgets/class-wp-widget-rss.php?marks=74#L74

and maybe

"Unknown action."
https://core.trac.wordpress.org/browser/trunk/src/wp-admin/comment.php?marks=329#L329

Also the (missing) period could be discussed in those strings.

[lukecavanagh]

Basic patch

[SergeyBiryukov]

Replying to Presskopp:

We made the "unknown post type" invalid, see #39171

The only reason for [39756] was to merge similar strings with the same meaning and remove duplicates. I don't see a benefit in changing other strings just to get rid of "unknown".

Moreover, an empty feed title doesn't make the feed invalid, so the existing string is more accurate.

[Presskopp]

I noticed there are much more instances of anything 'invalid' and only a few 'unknown' things. But following your input I think the ticket can be closed as wontfix.

[Presskopp]

let's close this, about the periods, see #42735