Call out that not all config values can be set via env vars

Author: weihanglo

src/doc/src/reference/config.md

@@ -190,8 +190,13 @@ variable.
 
 Environment variables will take precedence over TOML configuration files.
 Currently only integer, boolean, string and some array values are supported to
-be defined by environment variables. Descriptions below indicate which keys
-support environment variables.
+be defined by environment variables. [Descriptions below](#configuration-keys)
+indicate which keys support environment variables. Note that only with explicitly
+showing **"Environment: `ENV_VAR`"** a configuration value can be set in the
+form of environment variable. Those showing **"Environment: not supported"** or
+missing the entire **"Environment:"** field are not able to set via environment
+variables at this moment,
+due to [technicial issues](https://github.com/rust-lang/cargo/issues/5416).
 
 In addition to the system above, Cargo recognizes a few other specific
 [environment variables][env].

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

@@ -75,9 +75,10 @@ system:
 
 #### Configuration environment variables
 
-Cargo reads environment variables for configuration values. See the
-[configuration chapter][config-env] for more details. In summary, the
-supported environment variables are:
+Cargo reads environment variables for configuration values.
+Not all configurations can be set in the form of environment variables.
+See the [configuration chapter][config-env] for more details.
+In summary, the supported environment variables are:
 
 * `CARGO_ALIAS_<name>` — Command aliases, see [`alias`].
 * `CARGO_BUILD_JOBS` — Number of parallel jobs, see [`build.jobs`].