docs(ref): Find a place to comment on --cap-lints

Looking over the points of confusion highlighted in rust-lang#5998,
one is in rustc which is agnostic of any of this policy and the rest are
in historical documents.

Inspired by previous comments, I figured we could fit this into a
discussion of `[lints]` by talking about the scope of the feature.

Fixes rust-lang#5998

src/doc/src/reference/manifest.md

@@ -576,6 +576,11 @@ unsafe_code = "forbid"
 enum_glob_use = "deny"
 ```
 
+Generally, these will only affect local development of the current package.
+Cargo only applies these to the current package and not to dependencies.
+As for dependents, Cargo suppresses lints from non-path dependencies with features like
+[`--cap-lints`](../../rustc/lints/levels.html#capping-lints).
+
 ## The `[badges]` section
 
 The `[badges]` section is for specifying status badges that can be displayed