After deleting the key lib.plugin, modify the corresponding document.

Author: heisen-li

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

@@ -184,7 +184,6 @@ test = true            # Is tested by default.
 doctest = true         # Documentation examples are tested by default.
 bench = true           # Is benchmarked by default.
 doc = true             # Is documented by default.
-plugin = false         # Used as a compiler plugin (deprecated).
 proc-macro = false     # Set to `true` for a proc-macro library.
 harness = true         # Use libtest harness.
 edition = "2015"       # The edition of the target.
@@ -247,7 +246,7 @@ libraries and binaries.
 
 ### The `plugin` field
 
-This field is used for `rustc` plugins, which are being deprecated.
+This option is deprecated and unused.
 
 ### The `proc-macro` field
 

src/doc/src/reference/environment-variables.md

@@ -299,7 +299,7 @@ Cargo includes the following paths:
   Cargo to properly set the environment if additional libraries on the system
   are needed in the search path.
 * The base output directory, such as `target/debug`, and the "deps" directory.
-  This is mostly for legacy support of `rustc` compiler plugins.
+  This is mostly for support of proc-macros.
 * The rustc sysroot library path. This generally is not important to most
   users.