Documentation: Add a manual page about the new MemoryDevice file

Base/usr/share/man/man4/mem.md

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+## Name
+
+mem - physical system memory
+
+## Description
+
+`/dev/mem` is a character device file that is used by other programs to examine
+the physical memory.
+
+Trying to [`mmap`(2)](../mmap.md) a physical range results either with success,
+or with an error. When invoking [`mmap`(2)](../mmap.md) on bad memory range, 
+the kernel will write a message about it to the kernel log.
+
+By default, the kernel limits the areas which can be accessed. The allowed areas
+are the reserved ranges in physical memory, essentially limiting the access to
+ROMs and memory-mapped PCI regions on x86.
+
+To create it manually:
+```sh
+mknod /dev/mem c 1 1
+chmod 660 /dev/mem
+```
+
+## Returned error values after [`mmap`(2)](../mmap.md)
+
+* `EINVAL`: An access violation was detected.
+* `ENOMEM`: The requested range would wrap around, creating an access violation.
+
+## See also
+
+* [`mmap`(2)](../mmap.md)