Auto merge of rust-lang#6608 - rail-rain:note_on_as_conversions, r=phansch

Add a note to `as_conversions`

I have seen a couple of examples where there are some misunderstandings of `as_conversions` ([1](rust-lang/rust-clippy#5890 (comment)), [2](rust-lang/rust-clippy#6116 (comment)) and [3](rust-lang/rust-clippy#6428)). This PR adds the note that explains its purpose and relationship with other `as` related casts. Open question: should I list every related lints for discoverbility, or just suggest how to find these? I chose the former because there's no way to list only and all `as` related lints (e.g. on All the Clippt Lints, 'cast' includes some noises, but `cast_` excludes some) even though I cannot guarantee the list will be updated to include future changes.

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changelog: Add a note to the document of `as_conversions`

Author: bors

clippy_lints/src/as_conversions.rs

@@ -8,6 +8,14 @@ use crate::utils::span_lint_and_help;
 declare_clippy_lint! {
     /// **What it does:** Checks for usage of `as` conversions.
     ///
+    /// Note that this lint is specialized in linting *every single* use of `as`
+    /// regardless of whether good alternatives exist or not.
+    /// If you want more precise lints for `as`, please consider using these separate lints:
+    /// `unnecessary_cast`, `cast_lossless/possible_truncation/possible_wrap/precision_loss/sign_loss`,
+    /// `fn_to_numeric_cast(_with_truncation)`, `char_lit_as_u8`, `ref_to_mut` and `ptr_as_ptr`.
+    /// There is a good explanation the reason why this lint should work in this way and how it is useful
+    /// [in this issue](https://github.com/rust-lang/rust-clippy/issues/5122).
+    ///
     /// **Why is this bad?** `as` conversions will perform many kinds of
     /// conversions, including silently lossy conversions and dangerous coercions.
     /// There are cases when it makes sense to use `as`, so the lint is