For restriction lints, replace “Why is this bad?” with “Why restrict this?”

The `restriction` group contains many lints which are not about
necessarily “bad” things, but style choices — perhaps even style choices
which contradict conventional Rust style — or are otherwise very
situational. This results in silly wording like “Why is this bad?
It isn't, but ...”, which I’ve seen confuse a newcomer at least once.

To improve this situation, this commit replaces the “Why is this bad?”
section heading with “Why restrict this?”, for most, but not all,
restriction lints. I left alone the ones whose placement in the
restriction group is more incidental.

In order to make this make sense, I had to remove the “It isn't, but”
texts from the contents of the sections. Sometimes further changes
were needed, or there were obvious fixes to make, and I went ahead
and made those changes without attempting to split them into another
commit, even though many of them are not strictly necessary for the
“Why restrict this?” project.

1 files changed, 1 insertions, 1 deletions

diff --git a/clippy_lints/src/missing_assert_message.rs b/clippy_lints/src/missing_assert_message.rs
index 04df7b7a7e5..dd98352da86 100644
--- a/clippy_lints/src/missing_assert_message.rs
+++ b/clippy_lints/src/missing_assert_message.rs
@@ -10,7 +10,7 @@ declare_clippy_lint! {
     /// ### What it does
     /// Checks assertions without a custom panic message.
     ///
-    /// ### Why is this bad?
+    /// ### Why restrict this?
     /// Without a good custom message, it'd be hard to understand what went wrong when the assertion fails.
     /// A good custom message should be more about why the failure of the assertion is problematic
     /// and not what is failed because the assertion already conveys that.