Add symbol-addition to the how-to for new features (#1457)

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

src/feature-gates.md

@@ -9,7 +9,9 @@ modifying feature gates.
 See ["Stability in code"] for help with adding a new feature; this section just
 covers how to add the feature gate *declaration*.
 
-Add a feature gate declaration to `rustc_feature/src/active.rs` in the active
+First, add the feature name to `rustc_span/src/symbol.rs` in the `Symbols {...}` block.
+
+Then, add a feature gate declaration to `rustc_feature/src/active.rs` in the active
 `declare_features` block:
 
 ```rust,ignore

src/implementing_new_features.md

@@ -123,9 +123,9 @@ a new unstable feature:
 2. Pick a name for the feature gate (for RFCs, use the name
    in the RFC).
 
-3. Add a feature gate declaration to `rustc_feature/src/active.rs`
-   in the active `declare_features` block. See [here][add-feature-gate] for
-   detailed instructions.
+3. Add a feature gate declaration to `rustc_feature/src/active.rs` in the active
+   `declare_features` block, and add the feature gate keyword to
+   `rustc_span/src/symbol.rs`. See [here][add-feature-gate] for detailed instructions.
 
 4. Prevent usage of the new feature unless the feature gate is set.
    You can check it in most places in the compiler using the