On v3, treat all CLI variables as global variables

Closes #336
Ref #341

Co-authored-by: Egor Kovetskiy <e.kovetskiy@gmail.com>

docs/usage.md

@@ -488,19 +488,11 @@ $ TASK_VARIABLE=a-value task do-something
 
 Since some shells don't support above syntax to set environment variables
 (Windows) tasks also accepts a similar style when not in the beginning of
-the command. Variables given in this form are only visible to the task called
-right before.
-
+the command.
 ```bash
 $ task write-file FILE=file.txt "CONTENT=Hello, World!" print "MESSAGE=All done!"
 ```
 
-If you want to set global variables using this syntax, give it before any task:
-
-```bash
-$ task OUTPUT=file.txt generate-file
-```
-
 Example of locally declared vars:
 
 ```yaml