Add a few miscellaneous comments.

lib/codegen/src/isa/x86/abi.rs

@@ -217,6 +217,7 @@ fn callee_saved_gprs(isa: &TargetIsa) -> &'static [RU] {
     }
 }
 
+/// Get the set of callee-saved registers that are used.
 fn callee_saved_gprs_used(isa: &TargetIsa, func: &ir::Function) -> RegisterSet {
     let mut all_callee_saved = RegisterSet::empty();
     for reg in callee_saved_gprs(isa) {

lib/codegen/src/legalizer/call.rs

@@ -8,7 +8,8 @@ use flowgraph::ControlFlowGraph;
 use ir::{self, InstBuilder};
 use isa::TargetIsa;
 
-/// Expand a `call` instruction.
+/// Expand a `call` instruction. This lowers it to a `call_indirect`, which
+/// is only done if the ABI doesn't support direct calls.
 pub fn expand_call(
     inst: ir::Inst,
     func: &mut ir::Function,