docs: explain configuration before creating custom environments

docs/environment.md

@@ -11,6 +11,18 @@ Unless an environment is [chosen explicitly](#selection), Hatch will use the `de
 
 ## Creation
 
+The `env create` command creates environments from your project configuration; it does not add or modify that configuration. Before creating a custom environment, define it in one of your project configuration files:
+
+```toml tab="pyproject.toml"
+[tool.hatch.envs.env-name]
+```
+
+```toml tab="hatch.toml"
+[envs.env-name]
+```
+
+Then run `hatch env create env-name`. The `default` environment is available without an explicit configuration table.
+
 You can create environments by using the [`env create`](cli/reference.md#hatch-env-create) command. Let's enter the directory of the project we created in the [setup phase](intro.md#new-project):
 
 ```console

src/hatch/cli/env/create.py

@@ -15,7 +15,13 @@
 @click.argument("env_name", default="default")
 @click.pass_obj
 def create(app: Application, env_name: str):
-    """Create environments."""
+    """Create environments from project configuration.
+
+    Define custom environments in `pyproject.toml` under
+    `[tool.hatch.envs.ENV_NAME]` or in `hatch.toml` under `[envs.ENV_NAME]`
+    before running this command. It does not add or modify configuration.
+    The `default` environment is available without an explicit definition.
+    """
     app.ensure_environment_plugin_dependencies()
 
     environments = app.project.expand_environments(env_name)