Imprecise definition for proportional set size in documentation

[Albert Netymk]

>From https://www.kernel.org/doc/html/latest/filesystems/proc.html
```
The “proportional set size” (PSS) of a process is the count of pages
it has in memory, where each page is divided by the number of
processes sharing it.
```
The definition is a bit imprecise if a process uses multiple-mapping.
A real example is ZGC in OpenJDK, as asked at
https://superuser.com/questions/1485370/linux-misreports-process-size-with-heap-multi-mapping

A more precise wording could be something like `..., where each page
is divided by the number of mappings associated with it.`

PS: I have read
https://www.kernel.org/doc/html/latest/admin-guide/reporting-bugs.html,
but it's still unclear to me whether a mail or a ticket in bugzilla is
preferred.  Since this is just a documentation issue, maybe a mail is
fine.

/Albert