cgen: fixed array slice in function, add docs (#8481)

doc/docs.md

@@ -753,6 +753,33 @@ array_2 << array_1[..3]
 println(array_2) // [0, 1, 3, 5, 4]
 ```
 
+### Fixed Size Arrays
+V also supports arrays with fixed size. Unlike ordinary arrays, their 
+length is fixed, so you can not append elements to them, nor shrink them.
+You can only modify their elements in place. Note also, that most methods
+are defined to work on ordinary arrays, not on fixed size arrays.
+
+However, access to the elements of fixed size arrays, is more efficient,
+they need less memory than ordinary arrays, and unlike ordinary arrays,
+their data is on the stack, so you may want to use them as buffers if you
+do not want additional heap allocations.
+
+You can convert a fixed size array, to an ordinary array with slicing:
+```v
+mut fnums := [3]int{} // fnums is now a fixed size array with 3 elements.
+fnums[0] = 1
+fnums[1] = 10
+fnums[2] = 100
+println(fnums) // => [1, 10, 100]
+println(typeof(fnums).name) // => [3]int
+//
+anums := fnums[0..fnums.len]
+println(anums) // => [1, 10, 100]
+println(typeof(anums).name) // => []int
+```
+Note that slicing will cause the data of the fixed array, to be copied to
+the newly created ordinary array.
+
 ### Maps
 
 ```v