[env] Fix env_from docs and improve error messaging (#2386)

## Summary
See #2385 for context.

## How was it tested?
Not

docs/app/docs/configuration.md

@@ -182,11 +182,11 @@ Currently, you can only set values using string literals, `$PWD`, and `$PATH`. A
 
 ### Env From
 
-Env from takes a string or list of strings for loading environment variables into your shells and scripts. Currently it supports loading from two sources: .env files, and Jetify Secrsts.
+Env from takes a string for loading environment variables into your shells and scripts. Currently it supports loading from two sources: .env files, and Jetify Secrets.
 
 #### .env Files
 
-You can load environment variables from a `.env` file by adding the path to the file in the `env_from` field. This is useful for loading secrets or other sensitive information that you don't want to store in your `devbox.json`.
+You can load environment variables from a `.env` file by adding the path to the file in the `env_from` field (the file must end with `.env`). This is useful for loading secrets or other sensitive information that you don't want to store in your `devbox.json`.
 
 ```json
 {
@@ -198,15 +198,15 @@ This will load the environment variables from the `.env` file into your shell wh
 
 #### Jetify Secrets
 
-You can securely load secrets from Jetify Secrets by running `devbox secrets init` and creating a project in Jetify Cloud. This will add the `jetpack-cloud` field to `env_from` in your project.
+You can securely load secrets from Jetify Secrets by running `devbox secrets init` and creating a project in Jetify Cloud. This will add the `jetify-cloud` field to `env_from` in your project.
 
 ```json
 {
-    "env_from": "jetpack-cloud"
+    "env_from": "jetify-cloud"
 }
 ```
 
-Note that setting secrets securetly with Jetify Secrets requires a Jetify Cloud account. For more information, see the [Jetify Secrets](/docs/cloud/secrets/) guide.
+Note that setting secrets securely with Jetify Secrets requires a Jetify Cloud account. For more information, see the [Jetify Secrets](/docs/cloud/secrets/) guide.
 
 ### Shell
 

docs/app/docs/guides/secrets.md

@@ -28,9 +28,7 @@ For environment variables that you want to keep out of your `devbox.json` file,
 {
   "packages": {},
   "shell": {},
-  "env_from": [
-    "path/to/.env"
-  ]
+  "env_from": "path/to/.env"
 }
 ```
 

internal/devbox/devbox.go

@@ -32,6 +32,7 @@ import (
 	"go.jetpack.io/devbox/internal/devbox/envpath"
 	"go.jetpack.io/devbox/internal/devbox/generate"
 	"go.jetpack.io/devbox/internal/devconfig"
+	"go.jetpack.io/devbox/internal/devconfig/configfile"
 	"go.jetpack.io/devbox/internal/devpkg"
 	"go.jetpack.io/devbox/internal/devpkg/pkgtype"
 	"go.jetpack.io/devbox/internal/envir"
@@ -1009,9 +1010,9 @@ func (d *Devbox) configEnvs(
 		}
 	} else if d.cfg.Root.EnvFrom != "" {
 		return nil, usererr.New(
-			"unknown from_env value: %s. Supported value is: %q.",
+			"unknown env_from value: %s. Supported values are: \"%q\" or a path to a file ending in \".env\"",
 			d.cfg.Root.EnvFrom,
-			"jetpack-cloud",
+			configfile.JetifyCloudEnvFromValue,
 		)
 	}
 	for k, v := range d.cfg.Env() {

internal/devconfig/configfile/env.go

@@ -8,9 +8,11 @@ import (
 	"github.com/hashicorp/go-envparse"
 )
 
+var JetifyCloudEnvFromValue = "jetify-cloud"
+
 func (c *ConfigFile) IsEnvsecEnabled() bool {
 	// envsec for legacy. jetpack-cloud for legacy
-	return c.EnvFrom == "envsec" || c.EnvFrom == "jetpack-cloud" || c.EnvFrom == "jetify-cloud"
+	return c.EnvFrom == "envsec" || c.EnvFrom == "jetpack-cloud" || c.EnvFrom == JetifyCloudEnvFromValue
 }
 
 func (c *ConfigFile) IsdotEnvEnabled() bool {