Update file.md, precise read() return data size rules (#50)

* Update file.md, precise read() return data size rules

* Update file.md: more precise :read fn range description

src/rpcmethods/file.md

@@ -21,9 +21,9 @@ provided with various, but not all, types of file node modes. The methods
 |             | `*:crc` / `*:sha1` | `*:read` | `*:write` | `*:truncate` | `*:append` |
 |-------------|--------------------|----------|-----------|--------------|------------|
 | Read only   | ✔️                  | ✔️        | ❌        | ❌           | ❌         |
-| Fixed size  | ✔️                  | ✔️        | ✔️         | ❌           | ❌         |
-| Resizable   | ✔️                  | ✔️        | ✔️         | ✔️            | ✔️          |
-| Append only | ✔️                  | ✔️        | ❌        | ❌           | ✔️          |
+| Fixed size  | ✔️                  | ✔️        | ✔️         | ❌          | ❌         |
+| Resizable   | ✔️                  | ✔️        | ✔️         | ✔️          | ✔️         |
+| Append only | ✔️                  | ✔️        | ❌        | ❌           | ✔️         |
 
 ## `*:stat`
 
@@ -162,9 +162,11 @@ allow reading of the file.
 |------------|--------|
 | [Int, Int] | Bytes  |
 
-The parameter must be list with offset and size in bytes that identifies range
-to be read. The range can be outside of the file boundaries and in such case
-zero length bytes value is provided.
+The parameter must be a tuple containing `offset` and `size` in bytes 
+that identifies the range to be read. The implementation may 
+return less data than `size`, but it will never return 0 bytes if any data exists 
+at the specified offset. The range can be outside of the file boundaries and in such case
+zero length bytes value is returned.
 
 ```
 => <id:42, method:"read", path:"test/file">i{1:[0, 1024]}