docs(ref): Note that target-edition is deprecated (#15292)

### What does this PR try to resolve?

This is the first step in implementing rust-lang/rfcs#3772 (tracking
issue #15283)

### How should we test and review this PR?

Leaving off any talk of it being removed until we know the edition.

### Additional information

Author: weihanglo

src/doc/src/reference/cargo-targets.md

@@ -186,7 +186,6 @@ bench = true           # Is benchmarked by default.
 doc = true             # Is documented by default.
 proc-macro = false     # Set to `true` for a proc-macro library.
 harness = true         # Use libtest harness.
-edition = "2015"       # The edition of the target.
 crate-type = ["lib"]   # The crate types to generate.
 required-features = [] # Features required to build this target (N/A for lib).
 ```
@@ -268,14 +267,6 @@ to run tests and benchmarks.
 Tests have the [`cfg(test)` conditional expression][cfg-test] enabled whether
 or not the harness is enabled.
 
-### The `edition` field
-
-The `edition` field defines the [Rust edition] the target will use. If not
-specified, it defaults to the [`edition` field][package-edition] for the
-`[package]`. This field should usually not be set, and is only intended for
-advanced scenarios such as incrementally transitioning a large package to a
-new edition.
-
 ### The `crate-type` field
 
 The `crate-type` field defines the [crate types] that will be generated by the
@@ -313,6 +304,13 @@ name = "my-pg-tool"
 required-features = ["postgres", "tools"]
 ```
 
+### The `edition` field
+
+The `edition` field defines the [Rust edition] the target will use. If not
+specified, it defaults to the [`edition` field][package-edition] for the
+`[package]`.
+
+> **Note:** This field is deprecated and will be removed in a future Edition
 
 ## Target auto-discovery