Reword trivial_casts lint to better explain.

The current description of the trivial casts lint under the "allowed
by default" listing in the rustc book indicates the lint is for lints
which may be removed, which is less clear than saying it's for lints
which may be replaced by coercion (which is the wording used by the
error message included in the doc).

This commit changes the wording slightly to better describe what the
lint does.

Author: alilleybrinker

src/doc/rustc/src/lints/listing/allowed-by-default.md

@@ -232,7 +232,8 @@ error: lifetime name `'x` only used once
 
 ## trivial-casts
 
-This lint detects trivial casts which could be removed. Some example code
+This lint detects trivial casts which could be replaced with coercion, which may require
+type ascription or a temporary variable. Some example code
 that triggers this lint:
 
 ```rust