Doc comments on ipx-feed's and ipx-directory's public items (#354)

The style guide asks for doc comments on a library's public API. Every
type, function, constant and static the two libraries export now has
one, saying what it is for or what it does that its name does not:
fetch follows redirects itself so credentials stay on the feed's host,
Config::parse refuses repeated token names, Store::due_one is the
admin's Refresh. Fields and variants whose names say it all have none,
as the guide allows.

category_path's comment had slid above categories(), which then had
two; each has its own again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/feed/src/guard.rs

@@ -24,10 +24,12 @@ type BoxError = Box<dyn std::error::Error + Send + Sync>;
 pub struct Allowed(Arc<Vec<(IpAddr, u8)>>);
 
 impl Allowed {
+    /// Reads `fetch_internal`'s list, refusing an entry that is neither an address nor a network.
     pub fn parse(list: &[String]) -> Result<Self> {
         list.iter().map(|s| parse_net(s)).collect::<Result<Vec<_>>>().map(|v| Self(Arc::new(v)))
     }
 
+    /// Whether a connection to `ip` may go ahead: any public address, and the internal ones listed.
     #[must_use]
     pub fn permits(&self, ip: IpAddr) -> bool {
         let ip = canonical(ip);
@@ -109,6 +111,7 @@ pub fn is_internal(ip: IpAddr) -> bool {
 pub struct Guard(Allowed);
 
 impl Guard {
+    /// A guard letting through what `allowed` lists, besides the public internet.
     #[must_use]
     pub fn new(allowed: Allowed) -> Self {
         Self(allowed)
@@ -123,6 +126,7 @@ impl<S> tower::Layer<S> for Guard {
     }
 }
 
+/// The connector `Guard` wraps, checking each connection it makes.
 #[derive(Clone)]
 pub struct Guarded<S> {
     inner: S,