message: Update glossary for `feature`.
  Added the newly documented `target_feature` to help people disambiguate
  the various usages.
diff: @@ -21,15 +21,22 @@ manifest, and individual targets can specify which edition they use. See the
  ### Feature
  -A [*feature*][feature] is a named flag which allows for conditional
  +The meaning of *feature* depends on the context:
  +
  +- A [*feature*][feature] is a named flag which allows for conditional
  compilation. A feature can refer to an optional dependency, or an arbitrary
  name defined in a `Cargo.toml` manifest that can be checked within source
  code.
  -Cargo has [*unstable feature flags*][cargo-unstable] which can be used to
  -enable experimental behavior of Cargo itself. The Rust compiler and Rustdoc
  -also have their own unstable feature flags (see [The Unstable
  -Book][unstable-book] and [The Rustdoc Book][rustdoc-unstable]).
  +- Cargo has [*unstable feature flags*][cargo-unstable] which can be used to
  + enable experimental behavior of Cargo itself.
  +
  +- The Rust compiler and Rustdoc have their own unstable feature flags (see
  + [The Unstable Book][unstable-book] and [The Rustdoc
  + Book][rustdoc-unstable]).
  +
  +- CPU targets have [*target features*][target-feature] which specify
  + capabilities of a CPU.
  ### Index
  @@ -172,7 +179,7 @@ manifest is located.
  [cargo-unstable]: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html
  [config option]: reference/config.html
  [directory layout]: reference/manifest.html#the-project-layout
  -[edition guide]: https://doc.rust-lang.org/edition-guide/
  +[edition guide]: ../edition-guide/
  [edition-field]: reference/manifest.html#the-edition-field-optional
  [environment variable]: reference/environment-variables.html
  [feature]: reference/manifest.html#the-features-section
  @@ -184,6 +191,7 @@ manifest is located.
  [path overrides]: reference/specifying-dependencies.html#overriding-with-local-dependencies
  [pkgid-spec]: reference/pkgid-spec.html
  [rustdoc-unstable]: https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html
  +[target-feature]: ../reference/attributes/codegen.html#the-target_feature-attribute
  [targets]: reference/manifest.html#configuring-a-target
  [unstable-book]: https://doc.rust-lang.org/nightly/unstable-book/index.html
  [virtual]: reference/manifest.html#virtual-manifest
  