improve pagecache PSI annotations v2

[David Sterba]

On Thu, Sep 15, 2022 at 10:41:55AM +0100, Christoph Hellwig wrote:
> 
>  - spell a comment in the weird way preferred by btrfs maintainers

What? A comment is a standalone sentence or a full paragraph, so it's
formatted as such. I hope it's not weird to expect literacy either in
the language of comments or the programming language. The same style can
be found in many other kernel parts so please stop the nags at btrfs.