Add documentation of early parsing

Author: JonathanBrouwer

compiler/rustc_attr_parsing/src/attributes/diagnostic/on_unknown.rs

@@ -25,7 +25,7 @@ impl OnUnknownParser {
         self.span = Some(span);
 
         // At early parsing we get passed `Target::Crate` regardless of the item we're on.
-        // Only do target checking if we're late.
+        // Therefore, only do target checking if we can emit.
         let early = matches!(cx.should_emit, ShouldEmit::Nothing);
 
         if !early && !matches!(cx.target, Target::Use) {

compiler/rustc_attr_parsing/src/lib.rs

@@ -75,6 +75,13 @@
 //!
 //! This is equivalent to `#[repr(C, packed)]` and results in a single `AttributeKind::Repr`
 //! containing both `C` and `packed` annotations.
+//!
+//! ## Early attribute parsing
+//!
+//! While lowering to the HIR, all attributes are parsed using the attribute parsers.
+//! However, sometimes an attributes' parsed form is needed before the HIR is constructed.
+//! This is referred to as "early" attribute parsing,
+//! and is performed using the `parse_limited_*` family of functions on `AttributeParser`.
 
 // tidy-alphabetical-start
 #![feature(decl_macro)]