Auto merge of #12687 - ehuss:semver-remove-opt-dep, r=epage

SemVer: Update documentation about removing optional dependencies

This updates the documentation cautioning against removing optional dependencies with more up-to-date information about using the `dep:` syntax in the features table. This documentation was written before `dep:`, and I just forgot to update these docs when it was stabilized.

src/doc/src/reference/semver.md

@@ -2135,9 +2135,16 @@ std = []
 
 #### Possibly-breaking: removing an optional dependency {#cargo-remove-opt-dep}
 
-Removing an optional dependency can break a project using your library because
+Removing an [optional dependency][opt-dep] can break a project using your library because
 another project may be enabling that dependency via [Cargo features].
 
+When there is an optional dependency, cargo implicitly defines a feature of
+the same name to provide a mechanism to enable the dependency and to check
+when it is enabled. This problem can be avoided by using the `dep:` syntax in
+the `[features]` table, which disables this implicit feature. Using `dep:`
+makes it possible to hide the existence of optional dependencies under more
+semantically-relevant names which can be more safely modified.
+
 ```toml
 # Breaking change example
 
@@ -2152,7 +2159,33 @@ curl = { version = "0.4.31", optional = true }
 # ..curl removed
 ```
 
+```toml
+# MINOR CHANGE
+#
+# This example shows how to avoid breaking changes with optional dependencies.
+
+###########################################################
+# Before
+[dependencies]
+curl = { version = "0.4.31", optional = true }
+
+[features]
+networking = ["dep:curl"]
+
+###########################################################
+# After
+[dependencies]
+# Here, one optional dependency was replaced with another.
+hyper = { version = "0.14.27", optional = true }
+
+[features]
+networking = ["dep:hyper"]
+```
+
 Mitigation strategies:
+* Use the `dep:` syntax in the `[features]` table to avoid exposing optional
+  dependencies in the first place. See [optional dependencies][opt-dep] for
+  more information.
 * Clearly document your features. If the optional dependency is not included
   in the documented list of features, then you may decide to consider it safe
   to change undocumented entries.
@@ -2166,6 +2199,8 @@ Mitigation strategies:
   optional dependencies necessary to implement "networking". Then document the
   "networking" feature.
 
+[opt-dep]: features.md#optional-dependencies
+
 #### Minor: changing dependency features {#cargo-change-dep-feature}
 
 It is usually safe to change the features on a dependency, as long as the