add documentation

Author: perazz

doc/specs/stdlib_system.md

@@ -417,3 +417,39 @@ Returns one of the `integer` `OS_*` parameters representing the OS type, from th
 ```fortran
 {!example/system/example_os_type.f90!}
 ```
+
+## `null_device` - Return the null device file path
+
+### Status
+
+Experimental
+
+### Description
+
+This function returns the file path of the null device, which is a special file used to discard any data written to it. 
+It reads as an empty file. The null device's path varies by operating system:
+- On Windows, the null device is represented as `NUL`.
+- On UNIX-like systems (Linux, macOS), the null device is represented as `/dev/null`.
+
+### Syntax
+
+`path = [[stdlib_system(module):null_device(function)]]()`
+
+### Class
+
+Function
+
+### Arguments
+
+None.
+
+### Return Value
+
+- **Type:** `character(:), allocatable`
+- Returns the null device file path as a character string, appropriate for the operating system.
+
+### Example
+
+```fortran
+{!example/system/example_null_device.f90!}
+```