fix(kb/make): clarify inclusion of env files

knowledge base/gnu userland/make.md

@@ -13,6 +13,12 @@
 override override_me ?= default_value
 
 
+# Load env files.
+# If prefixed with '-' it does not error should any of the files not exist.
+include .env
+-include .env.local .env.extra
+
+
 # export variables for all programs to see
 # only exported in the commands' sub-shells
 export
@@ -39,11 +45,6 @@ previous_target:
 next_target:
 
 
-# load env files
--include .env
--include .env.local
-
-
 # conditionals
 ifeq "${shell uname}" "Darwin"
 expiration_date = ${shell date -v "+365d" "+%FT%TZ"}
@@ -82,7 +83,12 @@ apply: plan ${tf_plan_file}
 
 ## Load .env files in the Makefile
 
-Use this at top of a Makefile to export all variables in `.env`:
+Use one of those at the top of a Makefile to include and make available all variables in `.env`:
+
+```makefile
+include .env
+-include .env.local .env.extra
+```
 
 ```makefile
 ifneq (,$(wildcard ./.env))
@@ -91,15 +97,17 @@ ifneq (,$(wildcard ./.env))
 endif
 ```
 
-`ifneq` + `wildcard` is a typical way to check a file exists.<br/>
+`ifneq` + `wildcard` is a typical way to check a file exists.
+
 `include .env` imports `.env` into the Makefile variables.<br/>
+If prefixed with '-' (`-include`), it does not error nor warning should any of the included files not exist.
+
 `export` without parameters exports all variables set until now.
 
-
-
 ## Further readings
 
 - [Conditional syntax]
+- [Include]
 
 ### Sources
 
@@ -112,6 +120,7 @@ endif
 
 <!-- Upstream -->
 [conditional syntax]: https://www.gnu.org/software/make/manual/html_node/Conditional-Syntax.html
+[include]: https://www.gnu.org/software/make/manual/html_node/Include.html
 
 <!-- Others -->
 [how to load and export variables from an .env file in makefile?]: https://stackoverflow.com/questions/44628206/how-to-load-and-export-variables-from-an-env-file-in-makefile#70663753