Improve documentation for --env-set option flag

src/doc/rustc/src/command-line-arguments.md

@@ -508,16 +508,16 @@ encoded as UTF-8.
 <a id="option-env-set"></a>
 ## `env-set`: inject an environment variable
 
-This option flag allows to specify environment variables value at compile time to be
-used by `env!` and `option_env!` macros. It also impacts `tracked_env::var` function
-from the `proc_macro` crate.
+This option flag allows specification of environment variables at compile time for the
+`env!` and `option_env!` macros and `tracked_env::var` function from the `proc_macro`
+crate.
 
 This information will be stored in the dep-info files. For more information about
 dep-info files, take a look [here](https://doc.rust-lang.org/cargo/guide/build-cache.html#dep-info-file\
 s).
 
-When retrieving an environment variable value, the one specified by `--env-set` will take
-precedence. For example, if you want have `PATH=a` in your environment and pass:
+When retrieving an environment variable, the value specified by `--env-set` will take
+precedence. For example, if you want have `PATH=env` in your environment and pass:
 
 ```bash
 rustc --env-set PATH=env
@@ -529,20 +529,7 @@ Then you will have:
 assert_eq!(env!("PATH"), "env");
 ```
 
-It will trigger a new compilation if any of the `--env-set` argument value is different.
-So if you first passed:
-
-```bash
---env-set A=B --env X=12
-```
-
-and then on next compilation:
-
-```bash
---env-set A=B
-```
-
-`X` value is different (not set) so the code will be re-compiled.
+Crates will be fully re-compiled if `--env-set` arguments are changed.
 
 Please note that on Windows, environment variables are case insensitive but case
 preserving whereas `rustc`'s environment variables are case sensitive. For example,