Reword `trivial_casts` lint to better explain.

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.