Add various basic docstrings

src/platform/linux/mod.rs

@@ -65,6 +65,7 @@ pub fn e_raw(sys: usize) -> Result<usize> {
     }
 }
 
+/// Linux syscall implementation of the platform abstraction layer.
 pub struct Sys;
 
 impl Sys {

src/platform/mod.rs

@@ -1,3 +1,5 @@
+//! Platform abstractions and environment.
+
 use crate::{
     error::ResultExt,
     io::{self, Read, Write},
@@ -38,9 +40,11 @@ pub use redox_rt::auxv_defs;
 use self::types::*;
 pub mod types;
 
+/// The global `errno` variable used internally in relibc.
 #[thread_local]
 pub static ERRNO: Cell<c_int> = Cell::new(0);
 
+/// The `argv` argument available to a program's `main` function.
 #[allow(non_upper_case_globals)]
 pub static mut argv: *mut *mut c_char = ptr::null_mut();
 #[allow(non_upper_case_globals)]

src/platform/pal/mod.rs

@@ -25,6 +25,7 @@ mod signal;
 pub use self::socket::PalSocket;
 mod socket;
 
+/// Platform abstraction layer, a platform-agnostic abstraction over syscalls.
 pub trait Pal {
     fn access(path: CStr, mode: c_int) -> Result<()>;
 

src/platform/redox/mod.rs

@@ -74,6 +74,7 @@ macro_rules! path_from_c_str {
 
 use self::{exec::Executable, path::canonicalize};
 
+/// Redox syscall implementation of the platform abstraction layer.
 pub struct Sys;
 
 impl Pal for Sys {

src/platform/types.rs

@@ -1,3 +1,5 @@
+//! C data types for this platform.
+
 use core::i32;
 
 // Use repr(u8) as LLVM expects `void*` to be the same as `i8*` to help enable