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

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

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

Author: epage

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

@@ -189,7 +189,7 @@ harness = true         # Use libtest harness.
 crate-type = ["lib"]   # The crate types to generate.
 required-features = [] # Features required to build this target (N/A for lib).
 
-edition = "2015"       # The edition of the target.
+edition = "2015"       # The edition of the target (deprecated).
 ```
 
 ### The `name` field
@@ -310,9 +310,9 @@ required-features = ["postgres", "tools"]
 
 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.
+`[package]`.
+
+> **Note:* This field is deprecated.
 
 ## Target auto-discovery