Document that '&[u8]' is a form guard.

core/lib/src/form/from_form.rs

@@ -120,7 +120,7 @@ use crate::http::uncased::AsUncased;
 /// | IP Address | _inherit_ | **no default** | No | Yes | [`IpAddr`], [`Ipv4Addr`], [`Ipv6Addr`] |
 /// | Socket Address | _inherit_ | **no default** | No | Yes | [`SocketAddr`], [`SocketAddrV4`], [`SocketAddrV6`] |
 /// | [`TempFile`] | _inherit_ | **no default** | Yes | Yes | Data limits apply. See [`TempFile`]. |
-/// | [`Capped<C>`] | _inherit_ | **no default** | Yes | Yes | `C` is `&str`, `String`, or `TempFile`.  |
+/// | [`Capped<C>`] | _inherit_ | **no default** | Yes | Yes | `C` is `&str`, `String`, `&[u8]` or `TempFile`. |
 /// | [`time::Date`] | _inherit_ | **no default** | No | Yes | `%F` (`YYYY-MM-DD`). HTML "date" input. |
 /// | [`time::DateTime`] | _inherit_ | **no default** | No | Yes | `%FT%R` or `%FT%T` (`YYYY-MM-DDTHH:MM[:SS]`) |
 /// | [`time::Time`] | _inherit_ | **no default** | No | Yes | `%R` or `%T` (`HH:MM[:SS]`) |
@@ -628,6 +628,8 @@ impl<'v, T: FromForm<'v> + 'v> FromForm<'v> for Vec<T> {
  }
 }
 
+// impl_strict_from_form_field_from_capped!(Vec<u8>);
+
 #[doc(hidden)]
 pub struct MapContext<'v, K, V> where K: FromForm<'v>, V: FromForm<'v> {
  opts: Options,

core/lib/src/form/from_form_field.rs

@@ -28,11 +28,32 @@ use crate::form::prelude::*;
 /// }
 /// ```
 ///
+/// # Semantics
+///
+/// The implementation of `FromForm` for a `T: FromFormField` type operates as
+/// follows:
+///
+/// * When parsing is **strict**, the parser accepts the _first_ value or data
+/// field with the corresponding field name and calls `T::from_value()` or
+/// `T::from_data()` with the field's value, respectively. If more than one
+/// field value is seen, an [`ErrorKind::Duplicate`) is emitted. If no
+/// matching field is seen, an [`ErrorKind::Missing`] is emitted. Otherwise,
+/// the result from the call is emitted.
+///
+/// * When parsing is **lenient**, the parser accepts the first _expected_
+/// value or data field with the corresponding field name and calls
+/// `T::from_value()` or `T::from_data()` with the field's value,
+/// respectively. Unexpected values, identified by returning an
+/// [`ErrorKind::Unexpected`] from `from_value()` or `from_data()` are
+/// ignored. Any additional fields with a matching field name are ignored.
+/// If no matching field is seen and `T` has a default, it is used,
+/// otherwise an [`ErrorKind::Missing`] is emitted.
+///
 /// # Deriving
 ///
 /// `FromFormField` can be derived for C-like enums, where the generated
 /// implementation case-insensitively parses fields with values equal to the
-/// name of the variant or the value in `field(value = "...")`.
+/// name of the variant or the value in `field()`.
 ///
 /// ```rust
 /// # use rocket::form::FromFormField;